gaining weight the healthy way?!

I suffer from Orthorexia. If you're not familiar with the term, it's an eating disorder where you're too obsessed with eating healthily, you can look it up if you'd like since I'd rather not go too indepth with my problem. I'm 5'1 and 98 pounds. I consume about 900-1300 calories a day, which needs to change. I want to do it the healthy way though. slowly increasing my calorie intake while slowly adding more fat healthy fats, like mono and poly. I no longer have muscle and I'd like to. Any advice/tips on how to slowly and healthily gain weight but by muscle mass instead of fat mass??