Creating the Best French Onion Broccoli Cheese Casserole is simple when you follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C).
2. Sauté Onions: In a large skillet, heat the olive oil over medium heat. Add the sliced onions and sprinkle with salt. Sauté for about 10-15 minutes, stirring occasionally, until the onions are caramelized and golden brown.
3. Blanch Broccoli: While the onions are cooking, bring a pot of salted water to a boil. Add the broccoli florets and blanch for 2-3 minutes until bright green. Drain and set aside.
4. Mix Ingredients: In a large mixing bowl, combine the sautéed onions, blanched broccoli, sour cream, garlic powder, black pepper, cheddar cheese, and mozzarella cheese. Stir until well mixed.
5. Add Soup: Fold in the French onion soup into the mixture until evenly distributed.
6. Transfer to Baking Dish: Pour the mixture into a greased 9×13 inch baking dish, spreading it out evenly.
7. Prepare Topping: In a small bowl, mix the breadcrumbs with the melted butter. Sprinkle the breadcrumb mixture evenly over the casserole.
8. Bake: Place the baking dish in the preheated oven and bake for 30-35 minutes or until bubbly and golden brown on top.
9. Cool: Once done, remove from the oven and let it cool for about 5-10 minutes before serving.

These steps will lead you to a delicious and comforting casserole that is sure to be a hit!