A casserole is one of those dishes that doesn't have a clear definition. You know it when you see it. They are usually baked in a large, deep dish made of glass, ceramic, or enameled cast iron. A casserole is a loose collection of ingredients bound together in a dish and baked until they turrn into a cohesive whole. It is not quite a dip, and not quite a composed dish. Cheese is often involved, but not always. It's usually served hot, and you use a fork or spoon to eat it.
But aside from those basic principles, a casserole can be whatever you want. Any ingredient that can be baked can be part of a tasty casserole. That means that ice cream is out, but Doritos are totally in. Flavor profiles of casseroles can include everything from broccoli to garlic bread to ramen noodles. The only thing stopping you is what you have on hand and your imagination.
What you need to remember is that every casserole is made of a few crucial elements. Not every single casserole has all of them, but as a rule of thumb, here's what to look for: a starch, a binding agent or sauce, and a protein or vegetable. Many casseroles also have both vegetables and proteins, as well as a crunchy element to give some textural contrast, and cheese. Seasoning elements can be a part of the starch or the sauce, as well as the crunchy part. But that's pretty much all you need.
Watch: How to Make Italian Green Bean Casserole
Remember that when you put a casserole in the oven to bake, all the vegetables and meat should already be more or less cooked through. You can do this by roasting or sauteeing, or whatever method you prefer. The time in the oven has less to do with cooking the vegetables or meat and more to do with marrying the flavors together, crisping up the top, and setting the binder.
Let's take, for example, this Hoppin' John Casserole. Here, the starch is rice, so you cook the rice before adding it. The bacon, onion, and collards—your protein and vegetables—are cooked down before. The binding element and sauce is the egg and chicken stock you add tot he rice and bacon mixture. This casserole doesn't have cheese or a crunchy topping, but if you felt like adding something crunchy on top for a textural contrast—say a little crisped up and crumbled bacon after the casseorle is out of the oven—it would work perfectly. This Meditterranean Lamb Casserole works similarly. Rice is the main starch, and lamb and vegetables get cooked down and seasoned before combining that with tomatoes that are cooked down into a sauce. Here the binder is the tomato sauce and the yogurt added to the mixtue before it goes into the oven. The textural element is pita chips and feta, added toward the end of cooking and baked for about five minutes.
Once you have the basics down, you can get more creative with your ingredients. Can grits be your starch? Heck yes. That's what's happening in this Shrimp and Grits Casserole. Would garlic bread work? Yeah it would—like in this Pepperoni Pizza Casserole. You could use tater tots, or French fries, or noodles, or hash browns. It's really up to you.
Binding agents are a little less intuitive, but a couple beaten eggs works a treat in almost anything. You can always go with the classic can of mushroom soup approach for the sauce, or make a classic cheese sauce, or a bechamel, like in this Cauliflower-Goat Cheese Casserole. You're looking for something that'll impart some flavor and tie all the flavors together, so broth and tomato sauce are great options too.
When it comes to the crunchy topping, crumbled up chips work great. So do crispy onions orr crumbled bacon or breadcrumbs spread on top of the casserole about 5 minutes before it's done and then crisped up. Don't be shy, play around a little with your food. The best thing about casseroles is that they're pretty forgiving. If you mess it up, there's your old friend cheese, waiting in the wings to cover for you.