While there are a "million" articles out there on how to lose weight, and a bunch of tips on how to make that journey easier, I haven't seen a single useful article on how to gain weight.
Namely, functional weight.
Related - Gain Weight Without Getting Fat
I know it's hard to believe that someone would want to gain weight. We live in a weight conscious society. But, this is a challenge that many strength athletes face. They want to increase performance and get to the desired strength class so they can even things out when it comes to leverages and maximum power output.
The goal of this article is to give you easy-to-follow tips that are backed by science. They will allow you to manipulate your body in specific ways and make functional weight gain easier.
The main problem a person is faced with when attempting to gain weight on a "clean" diet is just the sheer volume of food that they have to consume to meet daily energy/macro goals.
When the main goal is functional weight gain, namely muscle mass gain, a dirty bulk is not really a good option. Since the ratio of muscle to fat that an individual gains isn't optimized it's easy to spiral out of control and end up in a "perma-bulk."
Thankfully there are a couple of tricks you can use to help you consume the needed food.
How to Gain Weight the Right Way
1. Using your body's insulin reaction to your advantage
Insulin is a peptide hormone. It consists of 51 amino acids that are linked together during the process of peptide synthesis.
Insulin is produced by the beta-cells of the pancreas and it's production is triggered by a blood glucose (sugar) level higher than 100mg/dL. "Insulin regulates orexigenic and anorexigenic peptides and body temperature." 
Compounds with orexigenic effects stimulate hunger. On the other hand compounds with anorexigenic effects inhibit it.
Thanks to the effects our body can regulate its food intake to accommodate our needs. Triggering an insulin response by taking in carbohydrates, especially sugars (sugars also have an orexigenic effect) can be used to promote hunger throughout the day and therefore increase total energy/macro intake.
This can be achieved through a high carb and lower protein breakfast (or first meal of the day). We want to keep protein intake lower since they take a longer time to digest. This will keep us feeling fuller for a longer period of time. of course, we want to feel hungry and be able to eat as soon as possible.
A great example of such a meal would be toast with some fruit and honey.
2. Regulation of veggies and fruit
This is the exact opposite of the way we would approach weight loss.
Fruits and especially vegetables carry a lot of volume and very few calories. They are what we call nutrient dense foods. While fruit can be a valuable tool to achieve the previously mentioned insulin response but veggies will fill you up fast and keep you full (thanks to the combination of high volume and high fiber content).
Even though they aren't ideal, in this case, we don't want to cut them out completely. They are too valuable.
We want to cut the recommended daily intake of 400g (roughly 14oz) in half and choose lower volume, lower fiber options.
During this time it is important to change up the vegetables on a regular basis and still try to respect the "5 a day rule" - 3 different types of veggies and 2 different fruits per day. It might even be a good idea to supplement with a quality multi-vitamin just to stay on the safe side.
3. Liquids, easy to eat meals and large meal = large appetite
The stomach is the first major destination of the food we eat. It is a hollow, muscular organ that is part of the GI system.
Contrary to popular belief it is not the place where the majority of digestion takes place. That would be the small intestines. Rather, it's a "holding tank" where the food is prepared for digestion.
A nearly empty stomach has a total volume of around 75ml, and a fasted one around 200ml (this applies to everyone obese or anorexic). The stomach has the ability to stretch to a total volume of a 1000ml. The stretching is not only there to allow us to eat more than a couple of tablespoons of food but it serves a crucial role in food intake regulation.
Ivan Petrovich Pavlov conducted a series of animal experiments and concluded that the act of our stomach stretching is a far more intense signal to the brain that we are getting full than the act of eating food itself. 
This signal transmission is rather slow when compared to other systems and reflexes in the body. This is the main reason it takes our body a little extra time to realize that we don't need any more food.
Managing liquid intake is important because most liquids you should be drinking (water, coffee, tea) are very low calorie, or don't have any calories at all. With that said, they do carry a significant amount of volume that will trigger the above-mentioned response.
This is the reason most people can simply lose weight by drinking a cup of water before eating. The water fills the stomach, stretching it in the process.
The body can't recognize the difference between water and food. All it knows is that the stretch means we are full. Drinking enough water is the key to health but over-hydrating during this period isn't beneficial.
Easy-to-eat meals will take the advantage of the slow response time of this reflex.
The main reason the majority of us are overeating is due to eating too fast and the brain not having time to give is the "STOP" signal. Making a risotto with veggies and adding your diced protein source into it will allow you to eat more food in a shorter period of time delaying the onset of satiety.
The brain-stomach connection works both ways. When food enters your mouth the brain send signals to the stomach allowing it to relax and preparing it to receive the food. This process is known as "gastric accommodation."
Your stomach will return to its original shape after the food passes through. Timing your meals so that it's never fully depleted should allow you to consume more food comfortably. In my experience eating larger meals will achieve just that.
Finding a Balance
Extremes in nutrition are never a good option. Make sure to adjust these tips to suit your needs and don't go overboard.
One more thing that won't be an option for top-level athletes but might be viable for some. You could slightly decrease your activity level and lower the number of calories you need to consume to gain weight.
The key during this period is to train and eat smart. Good luck.
References1) "Insulin Action in Brain Regulates Systemic Metabolism and Brain Function." PubMed Central (PMC), www.ncbi.nlm.nih.gov/pmc/articles/PMC4066341/.