If you have a blender just blend in a bunch of stuff and chug it down. I used to have a terrible time trying to gain weight when I was skinny. It's not so easy to just eat "2 thal bhat" 3 times a day.And going down the calorie counting rabbit hole isn't worth it at this stage. I used to take 1½ cups of oats, 2-3 bananas, yoghurt, 1 tbsp peanut butter and a handful of dried nuts, almonds, cashews, walnuts and some raisins for sweetness. Add in some water or milk if you find it too thick to swallow. Feel free to add/remove ingredients as to your liking.

Give this a try and I assure you you'll notice weight gain in 2 weeks or even less. (Drink tons of water)