Preheat an oven to 400F.
In a large mixing bowl, combine the sirloin, grill seasoning, allspice, grated onion and spaghetti. Work the mixture together with your hands until well combined. Roll mixture into meatballs slightly larger than a golf ball and arrange them on a baking sheet. Drizzle with about 2 tablespoons EVOO and pop into the oven. Bake until deep golden brown and cooked through, 15-18 minutes.
While the meatballs are baking, place a medium skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the chopped onion and garlic to the pan, season with salt and pepper, and cook until tender, 5-6 minutes. Add the tomatoes to the pan and bring up to a bubble. Add the basil to the pan and simmer the sauce until thickened, about 5 minutes.
Serve the meatballs in a serving dish nestled in the marinara sauce.