Growing up I was always super underweight from being a picky eater and just not a big eater in general. One day my gf made brownies and I ate waaay more than I usually would and felt nearly the fullest I've ever felt in my life.
Ever since then I've tried to eat until I'm completely full and then try to force in one or two more bites. It worked great for a while, I gained about 100lbs in a year and a half, but in the few years since then I've plateaued. I probably need to implement more snacking on top of big, filling meals.
I think to gain weight you have to push your body passed what you're used to on a somewhat regular basis, and when you're skinny you're REALLY not used to doing that kind of thing, but if you're into gaining, you'll find that it feels pretty good soon enough.
7 months