For 5 Serving(s)
1. Heat a generous drizzle of oil in a large, heavy skillet set over medium-high heat. Squeeze the sausage out of their casings into the pan at 1 inch intervals, forming meatballs. Add the onion, if you’re using it. Cook, rolling them around in the pan, until golden on all sides.
2. Add the garlic and cook for another minute, then add the lentils, tomatoes, tomato sauce and butter and bring to a simmer. Cook for 20 minutes, or until the lentils are very soft, the meatballs are cooked through and the sauce has thickened. Meanwhile, bring a large pot of salted water to a boil and cook the spaghetti according to package directions, or until al dente.
3. Serve the drained spaghetti topped with meatballs and sauce, with freshly grated Parmesan cheese on top.