While I don't have an eating disorder, I did want to put on about 20 lbs recently. I've changed my eating routine to have 5 -6 smaller meals each day. Some 'meals' may constitute a snack, truthfully.
Anyhow, if you can tolerate walnuts, they are a great source of essential fats along with Omega 3 and Omega 6 and a fourth cup has a whopping 200 calories.
I mix them with plain yogurt and blueberries as a mid-afternoon snack. I've gained about 15 pounds!