Preheat oven to 450°F (232°C). Grease a 9x13 inch baking dish with a little of the olive oil.
In a mixing bowl, combine the ground sirloin, bread crumbs, grated Parmesan, chopped parsley, dried oregano, crushed red pepper flakes, a pinch of kosher salt and pepper, 2 tablespoons olive oil, and the egg. Mix gently until just combined; do not overwork.
Coat your hands with a little olive oil and roll the mixture into 2-tablespoon meatballs (about 10–12). Place the meatballs in the prepared baking dish in a single layer.
Bake the meatballs for 15 minutes, until browned on the outside but still slightly undercooked inside.
Remove the dish from the oven, spoon the marinara sauce over the meatballs, sprinkle with shredded mozzarella, then return to the oven and bake another 15–20 minutes, until meatballs are cooked through and cheese is melted.
Alternatively, to make one large "giant" meatball, form the mixture into an oval, place in the prepared dish, bake 35–45 minutes until cooked through, and add the mozzarella in the last 10 minutes of baking.
Serve the meatballs and sauce over cooked whole wheat pasta or roasted spaghetti squash.
