Preheat your oven to 350°F (175°C) and butter a 9x13 inch baking dish.

Slice the yellow squash into 1/2-inch thick rounds and dice the onion into 1/4-inch pieces.

Melt 3.5 tablespoons of butter in a large skillet over medium-high heat. Add the sliced yellow squash and diced onion, seasoning with salt and pepper. Sauté for about 10 minutes until tender.

Drain the sautéed squash and onion mixture in a colander to remove excess moisture.

In a mixing bowl, whisk together the eggs, sour cream, cheddar cheese, Swiss cheese, mayonnaise, thyme, garlic powder, salt, and black pepper.

Fold the drained vegetable mixture into the creamy filling until coated, then spread it evenly in the baking dish.

Melt 3 tablespoons of butter and mix with crushed Ritz crackers and Parmesan cheese for the topping.

Sprinkle the cracker topping over the casserole and bake for 20-30 minutes until golden brown.

Let the casserole cool for 5-10 minutes before serving.