Combine the oil, onion, and garlic in a large Dutch oven or other heavy pot. Season with salt and pepper and set over medium heat. Cook, stirring occasionally, until the onion is just tender, about 5 minutes. Add the peppers, season with salt, and cook, stirring occasionally, until crisp-tender, 8 to 10 minutes. Add the sauce and bring to a simmer. Reduce the heat to low and simmer for 5 minutes.
While the sauce cooks, cook the spaghetti according to the package's directions. Drain well, then divide among serving plates. Top with the sauce, then the arugula. Grate parmesan on top and serve.
For an even meatier version, you can brown a pound of Italian sausage in the Dutch oven before adding the onion. You can swap any long pasta for the spaghetti. You can omit the arugula.