1-Preheat the oven to 375°F: Preheat the oven to 375°F.
2-Slice the yellow squash and zucchini into: Slice the yellow squash and zucchini into 1/4-inch rounds.
3-Heat the olive oil in a large: Heat the olive oil in a large skillet over medium heat and add the squash and zucchini.
4-Season with salt and black pepper and: Season with salt and black pepper and cook for about 5 minutes, stirring occasionally.
5-Add the chopped onion and minced garlic: Add the chopped onion and minced garlic to the skillet and cook until the onion is translucent.
6-Stir in the dried thyme and red: Stir in the dried thyme and red pepper flakes and remove the skillet from heat.
7-In a mixing bowl, combine the milk,: In a mixing bowl, combine the milk, shredded cheddar cheese, half of the Parmesan cheese, and melted butter.
8-Transfer the cooked vegetables to a greased: Transfer the cooked vegetables to a greased 9×13-inch baking dish and pour the cheese mixture over the top, stirring gently to coat.
9-Sprinkle the breadcrumbs and remaining Parmesan cheese: Sprinkle the breadcrumbs and remaining Parmesan cheese evenly over the casserole.
10-Bake for 25 to 30 minutes until: Bake for 25 to 30 minutes until the top is golden brown and the casserole is bubbly.
11-Let the casserole rest for 5 minutes: Let the casserole rest for 5 minutes before serving.