Lightly grease a 8"x12" casserole dish or a 2qt baking dish.
Arrange broccoli florets in the dish. Set aside.
In a medium mixing bowl, whisk together milk and cornstarch.
In a large saucepan, melt butter. Add onion and sautee until soft, about 2 minutes.
Gradually, add the milk mixture to the onions. Whisk after each addition. Let the sauce thicken before adding more milk. Repeat until you use all the milk.
Add the cheese and stir until melted.
Add the corn and stir until coated with sauce.
Pour the cheesy corn sauce over the broccoli.
Preheat the oven to 375 degrees F.
In a small measuring cup, mix melted butter and breadcrumb mix.
Sprinkle over the casserole.
Bake for 25 to 27 minutes, or until the topping is golden brown and the casserole is bubbling.
Let sit for 10 minutes before serving.
Refrigerate any leftovers for up to 2 days.
Notes
I like using cornstarch in sauces as it makes them smooth and it's easier to work with than flour.
Please note, that the nutrition value can vary depending on what product you use. The information below is an estimate. Always use calorie counter you are familiar with.