Preheat the oven to 400°F (200°C) and grease a large baking dish.
Cook ground beef and diced onion in a skillet over medium heat until browned; drain excess fat.
Add minced garlic, salt, pepper, paprika, onion powder, garlic powder, Worcestershire sauce, ketchup, mustard if using, and pickle relish; stir and simmer for 2 to 3 minutes.
Spread the beef mixture evenly in the prepared baking dish.
Sprinkle the shredded mozzarella or Monterey Jack and cheddar cheese evenly over the beef.
Layer the frozen French fries in a single layer on top of the cheese.
Scatter cooked bacon bits over the fries if using.
Bake uncovered for 30 to 35 minutes until the fries are golden brown and crispy.
Remove from the oven and let the casserole rest for 5 minutes.
Garnish with chopped parsley or green onions and serve with your favorite sauces.