Step 1

Bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente. Drain the spaghetti.

Step 2

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.

Step 3

Add the spaghetti to the sauce and toss over low heat until combined. Transfer to bowls and pass the cheese at the table.