In an unheated skillet large enough to hold the pasta later on, combine the oil, onion, garlic, red pepper flakes, bay leaf, and a pinch of salt, and toss to coat with the oil. Place over moderate heat, cooking just until the garlic turns golden but not brown, 2 to 3 minutes. Add the fennel, stirring so that the ingredients blend together and are evenly coated in the oil. Cover and sweat for 10 minutes over moderate heat. Toss the fennel with a wooden spoon from time to time to assure that none of the pieces stick or burn.Add the tomatoes and orange zest. Stir to blend and simmer uncovered until the sauce begins to thicken (about 15 min). Tast for seasoning, Remove and discard bay leaf.Meanwhile in a large pot bring 6 qts water to boil. Add 3 T salt and the penne, stirring to prevent the pasta from sticking to the bottom. Cook until al dente. Drain thoroughly. Combine with pasta and serve with freshly grated parmesan.
