Step 1

Bring a pot of salted water to a boil. Add the spaghetti and cook as the label directs. Reserve 1/2 cup cooking water, then drain.

Step 2

Meanwhile, heat 1 teaspoon olive oil in a large nonstick skillet over medium heat. Add the sausage and cook, breaking it up with a wooden spoon, until browned, about 5 minutes. Transfer to a bowl.

Step 3

Add the remaining 2 teaspoons olive oil to the skillet and increase the heat to medium high. Add the mushrooms, season with salt and pepper and cook until lightly browned, 3 to 4 minutes. Stir in the tomato paste and rosemary and cook, stirring, 1 minute. Return the sausage to the skillet, then add the cream and 1 cup water; bring to a gentle simmer and cook until slightly thickened, 8 to 10 minutes. Season with salt and pepper.

Step 4

Add the spaghetti to the skillet and toss with the sauce, adding some of the reserved cooking water to loosen, if needed. Top with shredded Parmesan, if desired.