Preheat Oven: Set your oven to 375°F (190°C).
Prepare Broccoli:
Cut broccoli into bite-sized florets.
Blanch in boiling water for 2–3 minutes, then transfer to an ice bath to stop cooking.
Drain and pat dry.
Make Cheese Sauce:
In a medium saucepan, melt butter over medium heat.
Stir in flour to create a roux, cooking for 1–2 minutes.
Gradually whisk in milk until smooth and thickened.
Reduce heat and stir in cheddar, Gruyère, garlic powder, salt, and pepper until melted and smooth.
Assemble the Casserole:
Spread a layer of blanched broccoli in a greased 9×13-inch baking dish.
Pour half the cheese sauce evenly over the broccoli.
Add another layer of broccoli and top with the remaining cheese sauce.
If desired, mix breadcrumbs with olive oil or melted butter and sprinkle over the top.
Bake: Place the dish in the oven and bake uncovered for 25–30 minutes, until the top is golden and edges are bubbly.
Cool: Let the casserole cool for 10 minutes before serving.