Step 1

Preheat Oven and Prep Pan: Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with cooking spray or butter to prevent sticking and ensure easy serving later.

Step 2

Sauté Aromatics: Heat a large skillet over medium heat. Add the finely chopped onion and minced garlic, cooking for 3-4 minutes until they become translucent and fragrant. This step builds the flavor foundation for your casserole.

Step 3

Cook Vegetables: Add the thinly sliced zucchini, yellow squash, and corn kernels to the skillet with the aromatics. Cook for approximately 5 minutes, stirring occasionally, until the vegetables are slightly tender but still have some bite to them. Remove the skillet from heat and set aside.

Step 4

Prepare Cheese Mixture: In a large mixing bowl, whisk together the beaten eggs and sour cream until smooth. Add the salt, black pepper, dried thyme, and red pepper flakes (if using). Stir in the shredded cheddar cheese and half cup of Parmesan until well combined.

Step 5

Combine Everything: Add the sautéed vegetable mixture to the cheese mixture, folding gently until all ingredients are evenly distributed. Pour this combined mixture into your prepared baking dish, spreading it out into an even layer.

Step 6

Make Crispy Topping: In a small bowl, mix together the panko breadcrumbs, melted butter, and remaining Parmesan cheese until the breadcrumbs are evenly moistened. Sprinkle this mixture evenly over the top of the casserole to create a crispy, golden crust when baked.

Step 7

Bake to Perfection: Place the casserole in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and crispy, and the casserole is bubbling around the edges. Insert a knife in the center to ensure it comes out clean.

Step 8

Rest and Serve: Remove from the oven and allow the casserole to rest for 5 minutes before serving. This resting period helps the casserole set up and makes it easier to portion.