1. Cook the onion with the olive oil in a medium, heavy-bottomed pot for about 5 minutes. A the mushrooms and cook an additional 10 minutes. Use your hands to break the sausage into smaller chunks and add to the pot. Cook for 10 to 15 minutes or until the sausage is cooked. Ad the tomato puree, warm water, red pepper, and salt. Simmer for 30 minutes over medium-low heat.In another large pot, bring water to a rolling boil. When the water is boiling, add salt and the penne pasta. Cook pasta until al dente, about 7 minutes, using the package instructions as a guide.
2. Remove and drain.
3. Put the sauce in a large frying pan and add the pasta. Stir over medium-high heat for 3 minutes and serve. Top each serving with Parmesan cheese, if desired.