Preheat your oven to 350°F (175°C) and grease a 9x13-inch casserole dish.

Slice the summer squash and zucchini into uniform rounds about a quarter-inch thick.

Melt about 3 tablespoons of butter in a large skillet. Sauté the onion for 3-4 minutes until translucent, then add minced garlic and cook for an additional 30 seconds.

Add the sliced summer squash and zucchini to the skillet and cook for about 5 minutes to soften slightly.

In a separate bowl, whisk together the eggs, milk, sharp cheddar, parmesan, thyme, salt, and pepper.

Pour the egg mixture over the sautéed vegetables, mix gently, and transfer to the greased casserole dish.

In a small bowl, combine crushed Ritz crackers with the remaining melted butter and sprinkle over the casserole.

Bake the casserole for 40-45 minutes until bubbly and golden brown.

Let the casserole rest for about 10 minutes before serving.