Casseroles cry out for improvisation. They follow a very basic formula, with nearly endless variety possible. Here’s a copy of an email I sent my non-cooking vegetarian friend who is trying to learn how to cook:
The general formula is:
Protein. Meat or cubed tofu (if you freeze and then thaw the tofu before cubing it, it’s a totally different texture, and holds up better in casseroles), tempeh, beans or other legume (Bean or pea like things that split into two halves), TVP or eggs - scrambled or hard-boiled. Cooked cubed chicken, beef, pork or ham, browned and drained ground beef or a drained can of tuna fish for omnivores.
Starch. Rice, barley, grains, noodles or ramen. These are mostly cooked first, before being added to the dish. (The ramen being an exception, because they’re not so much dry noodles as dehydrated noodles. They don’t need to “cook,” they just need to reabsorb water. They do this very quickly, so pre-cooking and then cooking again will get you mush.) The grain makes the legume option a complete protein. It also makes a cheap filler to give you more quantity of food and better satiety. Try packaged rice pilaf, or the grain/rice mixes in the bulk section.
Veggies. Anything, really. Small pieces are easiest. I use a lot of frozen veggies, because fresh stuff goes bad before we get to it around here, and it’s just a waste of money. Chopped celery is always good and adds a crunch. Peas are one of my favorite. Carrots, bell peppers, edamame (soy beans), green beans, corn. Lima beans if you’re into the nasty things. Sauteed onions or dried chopped onions go into nearly casserole I make.
Binder. Something moist to make it all stick together. “Cream of” soups, of course are fantastic for this. Mayonnaise, sour cream, cheese, milk (cheese, milk and beaten egg together with get you a quiche-like base for your casserole. Yummy!). For the vegans, there’s silken tofu. You can also make a quick sauce - melt butter, add an equal amount of flour and brown. Add stock, milk or cream and heat until thickened, stirring constantly.
I usually do one protein, one starch, two or three veggies and two or three binders. Mix ‘n’ match.
Smoosh it all together and heat. This can be done on the stove top, or, for a richer and more complex flavor, bake it at 350 until it’s bubbly. The last 10 minutes or so, add a:
Topping. Several options here:
Crunchy. Butter is your friend. Margarine if you must. Melted and mixed with a crumbly starchy cruchy thing, like crumbled crackers, stuffing mix, potato chips, or cornflakes. For the health conscious, buttered wheat germ is really good for a topping.
Biscuity. Whip up a biscuit recipe or boxed mix and spoon it on top of your casserole. Bake until browned. Also good with cornbread mix.
Crusty. Pie crust on top gives you a great pot-pie like thing. I never bother with a bottom crust, myself. I just top a plain-old casserole with pastry.
Potatoey. Shepherd’s pie is simply a ground beef casserole with whipped or mashed potatoes spread over the top and baked until brown. I highly recommend it.
(Toppings only work for the baked version. When in a pinch for time, however, I’ve been known to heat everything through on the stovetop and then shove it in the oven to brown a topping on top of it.)
So there you go. Most casseroles don’t need a whole lot of seasoning. Salt and pepper, maybe some garlic salt. Let the flavors of the food party together - don’t overwhelm them with herbs and spices.