Easy Zucchini Pizza Casserole

Preheat the oven to 400 degrees F.
Place zucchini noodles into a colander; sprinkle with salt; allow the zucchini to rest for 10 minutes in the colander; using your hands, wring out at much moisture as you can.
Now wrap the zucchini noodles in strong paper towels or cheesecloth, squeezing out any remaining moisture. (A yogurt straining bag, also known as a nut milk bag, works well for this.)
Next, combine the zucchini noodles with the eggs, Parmesan, and half of the mozzarella and cheddar cheeses; press this mixture into a greased 8 x 11 baking dish; bake for 20 minutes, uncovered, on the center oven rack.
Meanwhile, in a large skillet over medium heat, cook the ground beef with the onion flakes, Italian seasoning, and crushed garlic, crumbling the beef as it cooks until it is no longer pink and is fully cooked throughout. Drain any fats that accumulate, then stir the tomato sauce into the cooked ground beef.
Spoon the ground beef mixture over the zucchini mixture in the baking dish; sprinkle with the remaining cheeses, mini pepperoni, and diced bell pepper, if desired.
Bake the casserole, uncovered, on the center oven rack 25-30 minutes or until hot and bubbly throughout. Let the casserole rest for 5 minutes before serving.