Serves 3
6 Tbsp. extra-virgin olive oil
1 medium onion, finely chopped
Kosher salt
12 garlic cloves, thinly sliced
1/2 cup double-concentrated tomato paste
24 oz. mixed mushrooms (such as crimini, shiitake, oyster, and/or maitake), tough stems trimmed, coarsely chopped
1/2 cup heavy cream
Freshly ground black pepper
24 oz. rigatoni or other short pasta
3 Tbsp. unsalted butter
2 oz. Parmesan, finely grated (about 1 cup), plus more for serving
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.
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.
Add cream and 1.5 cups 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.
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.
Add pasta, butter, 1 cup pasta cooking liquid, and 2 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.
Divide pasta among shallow bowls or plates and top with more Parmesan.