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

In a large skillet over medium heat, melt 1 tablespoon of butter. Add the chopped onion and sauté until translucent, about 5 minutes.

Add the sliced yellow squash and zucchini to the skillet. Cook for an additional 5-7 minutes until the vegetables are tender. Stir in the corn and cook for another 2 minutes. Remove from heat.

In a large bowl, whisk together the milk, eggs, salt, black pepper, garlic powder, and paprika.

Add the sautéed vegetables to the bowl with the milk mixture. Stir in 1/2 cup of the shredded cheddar cheese and mix until well combined.

Pour the mixture into the prepared baking dish. In a separate bowl, combine the breadcrumbs with the melted butter and the remaining 1/2 cup of cheddar cheese. Sprinkle this mixture evenly over the casserole.

Bake in the preheated oven for 30-35 minutes, or until the top is golden brown and the casserole is set. Let it cool for 5 minutes before serving.