Step 1
Prepare dinner 12 oz. medium tube pasta in a big pot of boiling salted water, stirring sometimes, till al dente. Drain, reserving 1 cup pasta cooking liquid.
Step 2
In the meantime, warmth 2 Tbsp. extra-virgin olive oil and 2 Tbsp. unsalted butter in a big Dutch oven or different heavy pot or skillet over medium-high till butter is melted. Add 1 lb. crimini or button mushrooms, reduce into ¼” items, and cook dinner, stirring sometimes, till barely softened, about 2 minutes. Add 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt; cook dinner, stirring sometimes, till liquid is evaporated and mushrooms are browned, 5–7 minutes. Switch to a small bowl.
Step 3
Warmth remaining 2 Tbsp. extra-virgin oil in similar pot over medium-high. Add 8 oz. scorching Italian sausage and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt. Prepare dinner, breaking meat into smaller items with a wood spoon or heatproof rubber spatula, till cooked by means of, about 2 minutes.
Step 4
Add 1 giant crimson onion, finely chopped, and cook dinner, stirring sometimes, till frivolously browned, about 5 minutes. Add 4 garlic cloves, coarsely chopped, and cook dinner, stirring typically, till mixed and aromatic, about 2 minutes. Return mushrooms to pot and add 2 Tbsp. tomato paste and ¼ tsp. crushed crimson pepper flakes; season with freshly floor black pepper. Prepare dinner, stirring typically, till paste is darkened in colour, about 2 minutes. Pour in ⅓ cup balsamic vinegar and cook dinner, scraping up any browned bits caught to backside of pot, 30 seconds.
Step 5
Scale back warmth to medium. Add pasta, ½ cup reserved pasta cooking liquid, and remaining 2 Tbsp. unsalted butter. Prepare dinner, stirring typically and including extra pasta cooking liquid a little bit at a time if wanted, till pasta is coated and sauce is shiny, about 30 seconds.
Step 6
Switch pasta to a platter; prime with finely grated Parmesan.
