Chop the Vegetables: Chop 2 pounds mushrooms in batches with a knife or food processor and set aside. Chop 1 large onion, 2 medium carrots, 1 rib celery, and 3 cloves garlic the same way.
Start the Sauce: Heat 2 tablespoons extra virgin olive oil in a large pan. Add chopped onion, carrot, celery, garlic, 1 teaspoon dried oregano, and ½ cup tomato paste. Cook for 5 minutes, stirring until soft and fragrant. Add a splash of water if it sticks.
Cook the Mushrooms: Add chopped mushrooms, ¾ teaspoons salt, and pepper. Cook on medium-high for about 20 minutes, stirring sometimes, until all liquid is gone. Stir in 1 tablespoon balsamic vinegar, taste, and adjust.
Boil the Pasta: Cook 12 ounces fettuccine in salted boiling water until al dente. Save 1 cup of the cooking water, then drain.
Mix and Serve: Add pasta to the mushroom sauce with a splash of cooking water. Toss well, then top with 10 leaves fresh basil, a drizzle of olive oil, and parmesan.
