Over low-medium heat, in a large sauté pan, cook the onions in the oil until soft, about 4 minutes. Add the garlic and cook for another minute or so.
Stir in the tomato purée, Sriracha sauce, brown sugar, and salt.
Bring to a boil and then immediately reduce heat to low and simmer until the sauce thickens slightly, about fifteen minutes.
Then stir in the cilantro.
Add the prepared meatballs to the sauce, cover, and continue to simmer until the meatballs are cooked through, about 8 minutes. (About half way through the cooking process, remove the lid and gently move the meatballs around a bit.)
While the meatballs are cooking, cook pasta in a large pot of boiling salted water until it's al dente -- just tender but still firm to bite. Drain the pasta and then return it to the pot and drizzle it with olive oil. Cover and set aside.
When the meatballs are cooked, add them, along with the sauce, to pasta and toss to coat, taking care not to break the meatballs. (If you prefer, you can keep the meatballs and sauce separate and serve it over each portion of pasta.)