most airlines that fly long routes offer a ton of different menus: kosher, halal, vegetarian, vegan, asian vegetarian (my favorite - usually indian food, which still tastes good when microwaved as opposed to a lot of non-first-class airline meals), lactose-free, gluten-free, diabetic-friendly... many options available that are usually a cut above the typical "chicken or beef".
beware - you have to do this for each leg of a flight with layovers! i've occasionally had luck requesting a vegetarian or vegan meal in-flight but most of the time they only pack as many as are requested ahead of time.