Heat oil in a large pot over medium-high heat. Add the onion and cook for 5 minutes. Add the celery and cook for an additional 6–7 minutes.
Prepare the mushrooms:
Cut off the bottom half-inch of the mushrooms and discard them. Break the mushroom caps into small pieces. Shred the mushroom bodies into thin strips using a fork or your fingers. Set aside.
Cook the mushrooms:
Add the mushrooms to the pot and cook for 10 minutes—season with salt and pepper.
Blend the roasted bell peppers:
Transfer the roasted bell peppers to a food processor and blend until smooth.
Simmer the sauce:
Pour the roasted bell pepper puree into the sauce. Add the canned tomatoes and season with salt and pepper. Bring to a boil and then reduce heat to low. Cover and simmer for 45 minutes.
Cook the pasta:
If serving with pasta, cook the pasta according to package instructions while the sauce simmers—reserve 1/2 cup of the pasta cooking water.
Sauce the pasta:
Add the pasta cooking water to the sauce. Add the pasta and toss to coat for 2 minutes. Turn off the heat.
To serve:
Divide the pasta and mushroom sauce between shallow bowls. Garnish with fresh basil if you like. Enjoy!