This recipe adapted from Mario Batali's "Molto Italiano" (Ecco, 2005).
Bring 6 quarts of water to a boil in a large pot; season with salt.
Meanwhile, in a 10- to 12-inch saute pan, heat olive oil over high heat until smoking. Add onion; cook until light golden brown, about 5 minutes. Add mushrooms; cook until they release their water and it evaporates, 8 to 10 minutes. Add wine, tomato sauce, and butter; season with salt and pepper. Reduce heat to medium; cook until the consistency of a thick sauce, about 5 minutes.
Cook pasta in boiling water until tender, 1 to 2 minutes. Drain pasta, reserving about 1/4 cup of cooking water; add pasta to ragu. Toss over high heat for 1 minute to coat pasta, adding a few tablespoons of reserved cooking water if necessary
Divide pasta evenly among four warmed pasta bowls; garnish with Parmigiano-Reggiano. Serve immediately.