Broccoli Casserole

Name: Broccoli Casserole
Ingredients: 2 tablespoons extra-virgin olive oil, plus more for the baking dish; 4 heads broccoli, cut into bite-sized florets on the smaller side (about the size of a quarter); 8 ounces white button mushrooms, sliced; 2 teaspoons kosher salt; 1/2 teaspoon freshly cracked black pepper; 1 teaspoon onion powder; 1/4 cup (4 tablespoons) unsalted butter; 3 tablespoons all-purpose flour; 2 cups whole milk; 1 cup crushed butter crackers, such as Ritz; 1 cup freshly grated sharp Cheddar