: Heat oil in skillet over med low heat. Add onion & cook, stirring, til soft, sweet & slightly carmelized, about 10min. Add garlic & red pepper flakes & cook, stirring for 1min. to release fragrence. Add bell pepper, tomatoes & salt. Cover, adjust heat to maintain simmer, & stew, stirring occasinally, til vegetables are very soft, about 15min. : Transfer mixture to food processor or blender & puree, adding stock or water as needed to make a smooth sauce. Return to skillet. Reheat gently; taste & season w/S&P. Keep warm. : Meanwhile, cook penne in large pot of salted boiling water til al dente, 8-10min. Drain, then transfer to warm bowl. Add sauce & toss. Serve immediately on warm plates. See Eating Well,Oct95,pp68