In 2-quart saucepan, heat oil over medium heat. Add onion; cook about 1 minute.
Add garlic and tomatoes. Season to taste with salt and pepper. Stir in bell pepper, roasted peppers and pepper flakes. Cook about 20 minutes, crushing tomatoes with back of wooden spoon and stirring occasionally.
Pour sauce into blender or food processor. Cover; process with on-and-off pulses until smooth.