Preheat your oven to 375°F (190°C). Gather ingredients and equipment.

Melt 2 tablespoons of butter in a large skillet over medium heat. Add one chopped onion and sauté for 3-5 minutes.

Sprinkle in 3 tablespoons of flour over the sautéed onions, stirring continuously for one minute.

Gradually whisk in 2 cups of milk to the roux, cooking until thickened, about 4-5 minutes.

Fold in 2 cups of corn and 2 cups of diced yellow squash, adding salt and pepper to taste. Mix well.

Pour the mixture into a greased 9x13 inch baking dish, spreading it evenly. Top with remaining cheese and breadcrumbs if desired.

Bake for 25-30 minutes, until bubbly and lightly golden brown.

Let the casserole cool for a few minutes before serving.