I love casseroles for their versatility. If there is any ingredient here you don't have, swap it out for something you do have!
For example, this Chicken Fajita Casserole obviously has chicken in it. It also has chicken broth and cream of chicken soup.
To make the recipe vegetarian, swap out the chicken for mushroom, or eggplant, or tofu.
Use vegetable broth, and cream of mushroom soup instead!
Boom, tasty, and vegetarian.
If you're vegan, you can use your favorite creamy base. Perhaps cashew butter?
If you don't have rice, but you have quinoa instead, use quinoa instead! Although with quinoa I would recommend rinsing and cooking it before adding it to the recipe.
My husband really loved this Chicken Fajita Casserole, and we happily have leftovers to enjoy for the next couple days!