Have you tried eating a less clean diet to get your caloric intake up and see what kind of effects it has?
In response to your original question, obviously EVOO is a great one, peanut butter is a pretty good one too. I keep a jar at home and a jar at work and eat it by the spoonful throughout the day. Avocado, cheese, full-fat dairy; and although it could get expensive, drinks like Ensure are pretty healthy and have like 350 calories per 8oz servings. You can slam those back easily, even after eating. You can make your own trail mix and have it be pretty calorie-dense too, include stuff like raisins, almonds, dark chocolate, maybe some granola.