Inspired by classic renditions of Southern squash casserole recipes, this version maximizes flavor and keeps it super simple—which is exactly how a casserole should be! In our method, we slice the yellow squash (also known as crookneck squash) into round coins instead of grating it into a wet pile, which means you don’t need to spend time (and muscle) squeezing it over a colander to drain it of all that moisture. A good sear in a hot skillet with melted butter does that work for you, while also adding a layer of flavor to the often bland summer squash. This step makes all the difference in preventing a waterlogged casserole, so trust us, it’s worth it.
A brief sauté on the diced onion adds even more flavor to the base, which uses a combination of heavy cream and sour cream to achieve a gratin-like richness. Sharp cheddar cheese and Parmesan cheese complement the flavors of the cooked squash without overpowering it or pushing the dish into mac and cheese territory. But just in case you thought the decadence of the squash mixture wasn’t enough, we gild the lily by topping it off with a butter-cracker-crumb crust that gets nice and crispy in the oven. The final act to balance it all is a light dusting of lemon zest and fresh thyme for brightness.
The best squash casserole is one that’s shared, and this one is made to serve many as a side dish at a potluck or other gathering with friends and family. (It goes great with grilled pork chops!) If you’re taking it on the road, you can bake this casserole dish a couple hours ahead and reheat it for about 10 minutes at 350° before serving. And, if you can’t find yellow squash, use zucchini!