How to Make It
Bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente. Drain the spaghetti.
Meanwhile, in a large, deep skillet, heat the olive oil until shimmering. Add the ground beef and sausage and cook over moderately high heat, breaking up the meat with a spoon, until browned, about 6 minutes. Add the onion and garlic and cook over moderate heat until softened, about 4 minutes. Add the tomato paste and cook, stirring, until the meat is coated, about 3 minutes. Add the stock and rosemary and season with salt and pepper. Simmer the sauce over moderate heat until thickened, about 4 minutes. Add the milk and simmer for 2 minutes. Discard the rosemary sprig and season the sauce with salt and pepper.
Add the spaghetti to the sauce and toss over low heat until combined. Transfer to bowls and pass the cheese at the table.