Brown the ground beef together with onions & garlic, crumbling the meat with a spoon or spatula, until meat is fully cooked. Spoon out excess fat, if necessary, or soak up with a paper towel. Set meat aside.
While meat is browning, bring 6 cups of water to a boil in medium saucepan. Add broccoli & cook for 3-4 minutes; drain well.
Return saucepan to stove and reduce heat to low. Whisk together heavy cream, cream cheese, and parmesan cheese and stir constantly until cheese is melted. Add salt & pepper.
Mix together meat mixture, broccoli, and cheese, then add to greased 9x13 casserole dish; top with mozzarella cheese.
Bake at 375 degrees for 20-22 minutes or until cheese is melted and lightly browned. Sprinkle with red pepper flakes to taste.
