Preheat the oven to 450°F (232°C) and lightly grease a 9x13-inch baking dish with olive oil.
In a large 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.
Coat your hands with a little olive oil and roll the mixture into 2-tablespoon meatballs (about 10–12). Arrange the meatballs in the prepared baking dish, spacing them slightly apart.
Bake the meatballs for 15 minutes, until they are crisp on the outside but not fully cooked through.
Remove the dish from the oven, spoon the marinara sauce over the meatballs, sprinkle with shredded mozzarella, then return to the oven and bake an additional 15–20 minutes until the meatballs are cooked through and the cheese is melted.
Meanwhile, cook the whole wheat pasta according to package directions (or prepare roasted/spaghetti squash), drain, and divide among plates.
Serve the meatballs and sauce over pasta or spaghetti squash.
Optional: To make a single large "giant" meatball, shape the mixture into an oval, bake 35–45 minutes until cooked through, and add cheese during the last 10 minutes of baking.