Preheat the oven to 400° F (204° C).
Using a sharp knife, cut the top off of each bell pepper. Set tops aside. Remove seeds and membranes.
Use a paring knife to cut a jack-o-lantern face into each pepper. Save the pieces you cut off and chop them to use later.
Place the peppers in a baking dish with their tops on. Bake for 25 minutes. Remove from oven.
In a skillet, over medium high heat, add the ground beef, onions, and leftover chopped bell pepper. Saute until the meat is almost cooked through. Add the mushrooms and frozen cauliflower. Cook for 5 minutes.
Add the Rotel, salt, pepper, tomato sauce, chili powder, Italian seasoning, and red pepper flakes. Stir well to combine.
Fill the cooked peppers with the filling. Top with mozzarella cheese.
Place the stuffed peppers back in the oven without the tops until the cheese is melted and the peppers are warm through, about 10 minutes.