6 tablespoons butter, divided
1 onion, sliced
4 zucchini, sliced into 1/4-inch rounds
1 teaspoon salt
1/2 teaspoon ground black pepper
2 tablespoons flour
1 cup milk
3/4 cup Swiss cheese, shredded
3/4 cup breadcrumbs
Preheat the oven to 400 degrees Fahrenheit and grease a 9-by-13-inch casserole dish.
Melt 3 tablespoons butter in a large frying pan. Add onion and zucchini slices, and season with salt and pepper. Cook for about 10 minutes, until vegetables are soft.
Sprinkle flour over the vegetables, stir, and let cook for 2 minutes.
Stir milk in slowly to avoid lumping.
Bring to a simmer for 10 minutes, until thickened slightly.
Remove mixture from the heat and stir in Swiss cheese.
Melt remaining 3 tablespoons butter in a microwave on high for about 30 seconds.
Combine melted butter with breadcrumbs and stir.
Pour vegetable mixture into the prepared casserole dish and sprinkle butter-soaked breadcrumbs on top.
Bake for 20 minutes, until the entire casserole is hot throughout and breadcrumbs are toasted.