I don't find that this is ever a problem for me. YMMV, but I'd suggest eating calorie-dense foods such as nuts, beans, grains, and fatty foods.

This LiveStrong article has a sample meal plan:

A meal plan that provides over 3,000 calories begins with 1 cup of cooked quinoa topped with1/4 cup raisins, 1 oz. toasted almonds, 1 tbsp. honey and 1 cup hemp milk. For lunch, have 2 cups of whole wheat pasta tossed with 1 tbsp. olive oil, 1 cup white beans and sautéed kale. Dinner might include 6 oz. of firm tofu stir fried with broccoli, soy sauce, 1 oz. cashews and served over 1 cup brown rice. Snack on a peanut butter and jelly sandwich, made with whole grain bread, 2 tbsp. peanut butter and all-fruit spread; ½ cup of granola with soy milk and a smoothie made by blending a frozen banana with mangos, flaxseed meal, almond butter and coconut water.