Preheat oven to 350°F. In a small bowl, toss together bread crumbs and melted butter. Spread on a baking sheet. Bake until lightly browned, about 4 minutes.
In a 10-inch cast iron skillet (or other oven-safe skillet), cook bacon over medium heat until crisp, about 6-8 minutes. Use a slotted spoon to remove bacon to a paper towel-lined plate, reserving the drippings in the pan.
Add the butter to the skillet with the drippings. When the butter melts, stir in the onion and cook over medium-low heat until softened, about 10-12 minutes.
Add the zucchini and corn, cover with a lid, and cook until tender, about 10 minutes. Add the salt, pepper, nutmeg and garlic powder; cook uncovered for 1 more minute.
Stir in the flour and cook for 1 minute. Gradually add the milk or cream, stirring constantly. Cook over low heat for 2-3 more minutes, or until the sauce comes together and thickens.
Sprinkle the cheese over top.
Transfer the skillet to the 350°F oven. Bake (uncovered) for 20 minutes. Sprinkle buttered bread crumbs and bacon over top. Bake for 5 more minutes, or until the dish is bubbly.
Let stand for about 10 minutes before serving.
