Here are some energy-dense foods that are perfect for gaining weight:

Nuts: Almonds, walnuts, macadamia nuts, peanuts, etc.
Dried fruit: Raisins, dates, prunes and others.
High-fat dairy: Whole milk, full-fat yogurt, cheese, cream.
Fats and Oils: Extra virgin olive oil and avocado oil.
Grains: Whole grains like oats and brown rice.
Meat: Chicken, beef, pork, lamb, etc. Choose fattier cuts.
Tubers: Potatoes, sweet potatoes and yams.
Dark chocolate, avocados, peanut butter, coconut milk, granola, trail mixes.
-Healthline.com