I don't understand what もいい means in the following sentence:
It's ても + いい rather than て + もいい. ても (or でも) means "even though" or "even if". This いい is just "good" or "fine".
So ローストしてもいい literally means "even if you roast, it's good". Practically you can think of てもいい simply as "can". The sentence says you can either stir-fry, grill or roast chicken, and they're all good.
し after いい is used to list two or more clauses. See: し grammar question