Place bell peppers on an aluminum foil-lined baking sheet.
Broil 5 inches from heat (with electric oven door partially open) 5 to 10 minutes on each side or until bell peppers look blistered.
Place bell peppers in a heavy-duty zip-top plastic bag; seal and let stand 10 minutes to loosen skins. Peel bell peppers; remove and discard seeds.
Sauté sliced mushrooms, chopped purple onion, and garlic in 1 1/2 teaspoons hot oil in a large skillet 10 minutes or until liquid has evaporated. Remove mixture from heat, and set aside.
Process bell peppers in a food processor until smooth, stopping to scrape down sides. Add mushroom mixture, remaining 1 1/2 tablespoons oil, Parmesan cheese, and next 8 ingredients; pulse until ground. Serve with breadsticks, crackers, or assorted fresh vegetables. Garnish, if desired.