Yellow Squash Casserole

Preheat oven to 375°. Grease a 13x9" baking dish with unsalted butter. Melt 1 Tbsp. unsalted butter in a large skillet over medium-high heat. Working in 2 batches and adding 1 Tbsp. unsalted butter between batches, cook 3 lb. yellow summer squash, sliced crosswise ¼" thick, tossing often, until softened and golden brown, 8–10 minutes. Season each batch with 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt and transfer to a large bowl.

Melt 1 Tbsp. unsalted butter in same skillet. Add 1 medium onion, finely chopped, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt. Cook, stirring often, until onion is softened and golden brown, about 5 minutes. Reduce heat to low and pour in ¾ cup heavy cream; stir to combine. Bring to a gentle simmer, then remove from heat and stir in ½ cup sour cream and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt.

Add mixture to bowl with squash, then add 4 oz. sharp white cheddar, grated (about 1 cup), ⅓ cup finely grated Parmesan, 1 Tbsp. thyme leaves, and ½ tsp. freshly ground black pepper; stir gently to combine. Transfer to prepared baking dish and spread into an even layer. Wipe and reserve skillet.

Using your hands, crush 1 sleeve Ritz crackers or other buttery crackers into coarse crumbs in a medium bowl. Melt remaining 3 Tbsp. unsalted butter in reserved skillet over medium heat. Stir in remaining ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt until dissolved, then pour over cracker crumbs. Sprinkle remaining ⅓ cup finely grated Parmesan over and toss to combine. Scatter cracker mixture evenly over casserole.

Bake casserole until golden brown on top and bubbling, 35–40 minutes. Top with zest of 1 lemon and more thyme leaves. Let cool 5 minutes before serving.