Creating Brussels Sprout Casserole is simple if you follow these steps:

1. Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with butter or cooking spray.
2. Prepare Brussels Sprouts: In a large pot of boiling salted water, blanch the halved Brussels sprouts for about 5 minutes. Drain and set aside.
3. Sauté Vegetables: In a skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes. Stir in the minced garlic and cook for an additional minute.
4. Combine Ingredients: In a large mixing bowl, combine the blanched Brussels sprouts, sautéed onion and garlic, heavy cream, cheddar cheese, Parmesan cheese, salt, black pepper, and crushed red pepper flakes (if using). Mix well until everything is evenly coated.
5. Transfer Mixture: Pour the Brussels sprout mixture into the prepared baking dish, spreading it out evenly.
6. Add Topping: In a small bowl, combine the breadcrumbs with a tablespoon of olive oil. Sprinkle the breadcrumb mixture evenly over the top of the casserole.
7. Bake: Place the casserole in the preheated oven and bake for 30-35 minutes, or until the top is golden brown and the casserole is bubbly.
8. Cool: Remove from the oven and let it cool for about 5-10 minutes before serving.
9. Garnish: If desired, sprinkle chopped fresh parsley on top for added color and flavor.

By following these steps, you will create a Brussels Sprout Casserole that is both delicious and visually appealing.