On the weekend, I will bake 8 or so large chicken breasts, or cook several (4-5) pounds of beef heart (ground or sliced). I might toss in an onion or just some spices. Then I will boil 5+ lbs of potatoes or yuca. I split this all up for my post workout lunches throughout the week. Very basic and utilitarian, but easy and palatable enough for me to gain weight on.
I also drink a couple glasses of raw milk on workout days and eat some Greek yogurt.