To gain weight in a healthy way, focus on eating calorie-rich but nutritious foods and combining them with strength training. Include foods like whole wheat roti or rice, dal, eggs or paneer, milk, curd, nuts (almonds, peanuts), peanut butter, bananas, and healthy oils like ghee in moderate amounts. Add protein sources to every meal to build muscle, and consider a homemade smoothie with milk, banana, peanut butter, and dry fruits. Start light weight training 3-4 times a week to ensure weight gain is mostly muscle, not fat. If you are unable to gain weight despite good intake, consult a doctor to rule out underlying issues.