That's the question.I'm also severely underweight.Height:185cm Weight:53kg.IBD still makes me drop pounds.
As someone with Crohns I find the lower the frequency of my meals the better. That being said, I also find the smaller the volume of my meals the better I feel. You should start with the SCD intro diet or the intro GAPS protocol. I honestly would stay away from veggies except puréed carrots or spinach until you've made progress, anything that you're eating and cannot absorb just leaves undigested fermentable materials in your small intestine that can feed harmful bacteria (or bacteria in unhealthy ratios).
http://www.breakingtheviciouscycle.info/beginners_guide/the_intro_diet2.htm ^^ read this and Robb Wolf's autoimmune protocol and start with a combination that makes you feel best. Just remember that you have the equivalent of open sores inside your intestines and smallest volume of food that you can have passing over these sores is what will allow healing. That means maximum nutrition needs to be derived from that small volume!
Start slow with the fats, they will be more easily digested in a few months.
First, make sure your diet is super clean - follow Robb Wolf's autoimmune protocol.
Once you're diet is dialed in, then we can optimize. Generally, punctuated eating (few meals, no snacks) is good. However, since you're recovering from gut damage and malnutrition, things may be different. Eat meals that aren't so big that you stress your gut, then eat as many times a day as you need to to feel satiated and recover weight. After your gut heals you can slowly transition to fewer bigger meals.
I still have Chrohns. And have been paleo/primal for close to two years. Its possible it might not heal entirely. But it will improve a lot. Find what your trigger foods are though, and eat as clean as possible until you can reintroduce more foods. I don't drop weight from Chrohns at all and tend to be constipated. I have a redundant colon so that could be a cause of the stubborn constipation. If you eat only two meal a day it better be large meals if your loosing weight. Or very high calorie. Yogurt might help with diarrhea.