Chop and saute onion in butter until soft (along with mushrooms if using). Add garlic and saute for about one minute.
Add all remaining ingredients and simmer for at least 5 minutes but you'll have the best flavor with around 20 minutes.
Use with your favorite pasta!
For best results, save 1/2 cup of the cooking water from the pasta. Once the pasta is drained, put it back in the pot and stir in the 1/2 cup water and one cup of the sauce. Let sit for just a couple of minutes before serving.Top with hot sauce and fresh Parmesan.