Preheat the Oven: Preheat your oven to 400°F (200°C). Grease a 9×13-inch baking dish.

Cook the Beef: In a large skillet, heat the olive oil over medium heat. Add the ground beef and cook, breaking it apart, until browned and fully cooked, about 6–8 minutes. Drain any excess fat.

Add Seasonings and Sauce: Add the chopped onion and garlic to the skillet and cook until softened, about 3–4 minutes. Stir in the salt, pepper, ketchup, and mustard. Let the mixture simmer for another 2–3 minutes, then remove from heat.

Assemble the Casserole: Spread the frozen French fries evenly in the prepared baking dish. Pour the beef mixture over the fries, spreading it evenly.

Add the Cheese: Pour the milk over the casserole, then top with shredded cheddar and mozzarella cheese (if using).

Bake: Bake for 25–30 minutes, or until the cheese is melted and the fries are golden and crispy.

Garnish and Serve: If desired, garnish with diced pickles before serving.