This gratin is actually like a really thick, cheesy chowder and the flavorful cheese sauce is definitely what makes the dish. The vegetables sort of melt down into the sauce, while breadcrumbs and a sprinkling of cheddar form a nice crust on top. It’s an easy recipe to make too, the only thing to note is that you’ll want to factor in 15 minutes for the summer squash to release their excess water; just slice the squash, salt it, and let it sit in the sink to do its thing while you work on the rest of the recipe.
Butter, to grease the dish
2 pounds (about 4 medium-large) summer squash, preferably a mix of zucchini and gold bar
1 1/2 teaspoons salt, divided
3 tablespoons unsalted butter
1 very large onion, diced
3 large cloves garlic, crushed
4 tablespoons all-purpose flour
2 cups whole milk, at room temperature
1/2 teaspoon sweet paprika
1/2 teaspoon black pepper
8 ounces (about 2 cups) shredded cheddar cheese
2 tablespoons chopped fresh parsley (optional)
1 tablespoon Italian-style breadcrumbs
Preheat the oven to 400F; grease a 2.5-liter casserole dish with butter and set aside.
Slice the summer squash into 1/4-inch-thick circles, and then put it into a colander in the sink and sprinkle on 1 1/4 teaspoons salt. Gently toss so all the squash is coated with salt. Leave the squash to sit in the sink for 15 minutes so it releases some of its water, then rinse it under cool running water, drain well, and pour it out onto a paper towel-lined baking sheet. Gently pat the squash dry with paper towels.
Meanwhile, heat the butter in a 5-quart pot over medium heat. Add the onion and cook until softened but not browned, about 8 minutes, stirring occasionally. Add the garlic and cook 30 seconds more, stirring constantly.
Whisk in the flour and cook 1 minute, then gradually whisk in the milk. Stir in the paprika, black pepper, and remaining 1/4 teaspoon salt. Bring it up to a gentle boil, stirring frequently, and then turn off the heat and whisk in 2/3 of the shredded cheddar. Add the summer squash and parsley (if using), and gently stir to coat the squash.
Pour the summer squash into the prepared casserole dish. Sprinkle the remaining 1/3 of the shredded cheddar on top, and sprinkle on the breadcrumbs.
Bake until the casserole is golden and the squash is tender, about 20 minutes.