Preheat the Oven: Preheat your oven to 400°F (204°C)
Prepare the broccoli: Boil the broccoli 7-10 minutes.
Prepare the cauliflower: Microwave the cauliflower for 4-5 minutes.
Prepare the sauce: Melt 4 tbsp of butter in a pan. Add the garlic, and sauté. Add the mushrooms to the pan and cook until tender. Add in the heavy cream, and the salt and pepper. Simmer for about 5 to 7 minutes
Blend the soup: Pour the cream of mushroom into a blender and blend until smooth.
Mix the cheese: In a bowl combine the cheddar cheese, and the Monterrey jack.
Combine the Ingredients: In a large bowl, combine the steamed cauliflower rice, the broccoli florets, the blended cream of mushroom soup, 1 1/2 of the cheese, and the egg.
Prepare a baking dish: Butter the bottom and the sides of a large baking dish and pour the Broccoli and Cauliflower Rice Cheese Casserole into the dish. Top with the remaining cheese
Bake in preheated oven: Bake the Broccoli and Cauliflower Rice Cheese Casserole at 400°F (204°C) for 30-35 minutes until bubbly around the edges.
Serve: Spoon the Broccoli and Cauliflower Rice Cheese Casserole into serving dishes.
Enjoy: Serve the Broccoli and Cauliflower Rice Cheese Casserole hot, and enjoy your low-carb, comfort food-inspired meal.