Step 1

Heat oil in a small pot or large saucepan over medium. Add onion and a big pinch of salt. Cook, stirring occasionally, until onion is very soft, 6–8 minutes. Add garlic and cook, stirring often, until softened and beginning to turn golden around edges, about 4 minutes.

Step 2

Add tomato paste to pot and cook, stirring constantly, until slightly darkened in color, about 1 minute. Add mushrooms and season with more salt. Cook, stirring often, until softened, 6–8 minutes.

Step 3

Add cream and 1 cup water, increase heat to high, and bring to a boil. Reduce heat and simmer, stirring occasionally, until ragù is slightly thickened, 13–17 minutes. Taste and season with pepper and more salt as needed.

Step 4

Cook pasta in a large pot of boiling salted water, stirring occasionally, until very al dente, about 2 minutes less than package directions. Drain, reserving 2 cups pasta cooking liquid.

Step 5

Add pasta, butter, 1 cup pasta cooking liquid, and 1 oz. Parmesan to ragù. Cook, tossing and adding more pasta cooking liquid by ¼-cupful as needed, until sauce coats pasta and pasta is al dente, about 2 minutes.

Step 6

Divide pasta among shallow bowls or plates and top with more Parmesan.