Hint: It's in the name of the dish.
I’m not going to bury the lede and keep you waiting for the answer—not while the very best summer squash is in season right this very moment. The one ingredient I would never, ever put in squash casserole is squash. Boiled squash, that is.
A Southern-style squash casserole is so rich and creamy that you might as well be eating dessert. Typically found on meat-and-three menus and on dinner tables across the South, it is typically loaded with cheese, eggs, sour cream, mayonnaise, and oh, yeah—yellow squash, also called crookneck squash. While the squash hiding in that luscious filling isn’t the reason why Southerners find themselves taking second and third helpings, it is the foundation of the recipe, and can make or break the dish. If you’ve ever taken a bite of mushy squash in a thin, watery casserole, you know what I mean. Ugh.
Squash are loaded with water, which is why they are typically cooked twice in a squash casserole recipe: once on the stovetop, and then in the oven, in the actual casserole. The goal is to remove as much liquid as possible during the initial cooking process to prevent the liquid in the squash from watering down the actual casserole. When developing our best-ever Old-School Squash Casserole our test kitchen cooked the squash several different ways to find out which method resulted in tender (but not mushy) squash that holds its shape.
Watch: Two-Cheese Squash Casserole
Testers discovered that sautéing—not boiling—the squash was the best way to remove liquid without overcooking the squash. You don’t want to brown the squash, just cook it until it is tender and the liquid has evaporated. Then, to make sure as much water is removed as possible, transfer the cooked squash to a colander to drain for five minutes. Boiling not only infuses the squash with extra liquid, it also overcooks the squash, making it way too soft once it bakes in the oven.
Whether you’re using fresh or frozen squash to make this casserole, be sure to sauté the squash until it is just tender and never, ever boil it.