TOP SECRET!!!!Nuts. It don't matter what kind. They're all good for you. Walnuts, almonds, sunflower seeds, cashews, peanuts, etc. 1 cup will add about 700-800 calories to your diet.
Put peanut butter on EVERYTHING! Pancakes, mashed potatos, oatmeal.
Add olive oil to everything. 1 TBSP is good for about 100 cal.
Or, just get a bigger fork.