2 tablespoons olive oil
1/2 medium yellow onion, finely chopped
1 clove garlic, minced
Freshly ground black pepper
1 (24- to 26-ounce) jar marinara sauce (about 3 cups)
1 pound precooked meatballs (about 12 (1 1/2-inch) meatballs), thawed if frozen
6 medium zucchini (about 2 1/4 pounds total), ends trimmed
Grated Parmesan cheese, for serving
Heat the oil in a large pot over medium heat until shimmering. Add the onion and garlic, season with salt and pepper, and cook, stirring occasionally, until softened, about 5 minutes.
Add the marinara sauce and meatballs and stir to combine. Simmer, stirring occasionally, until the meatballs are heated through and the sauce is slightly thicker, 12 to 15 minutes. While the meatballs are simmering, spiralize the zucchini.
Add the spiralized zucchini and cook, tossing the noodles often in the sauce, until the zucchini just starts to soften, about 5 minutes. (The zoodles will look dry in the beginning, but will eventually release a lot of water that thins out the sauce.) Serve immediately, topped with Parmesan cheese.