Peel and chop the onion to measure 1/2 cup. Peel and finely chop the garlic. Cut the bell pepper in half lengthwise, and cut out seeds and membrane. Chop enough bell pepper to measure 1/4 cup. Wrap any remaining bell pepper with plastic wrap and refrigerate.
In a 2-quart saucepan, heat the oil over medium heat 1 to 2 minutes. Add the onion, garlic and bell pepper. Cook 2 minutes, stirring occasionally.
Stir in the tomatoes with their liquid, tomato sauce, basil, oregano, 1/4 teaspoon salt, the fennel seed and pepper. Heat to boiling over high heat. Once mixture is boiling, reduce heat just enough so mixture bubbles gently and does not spatter.
Cover with lid; cook 35 minutes, stirring about every 10 minutes to make sure mixture is just bubbling gently and to prevent sticking. Lower the heat if the sauce is bubbling too fast.
After the sauce has been cooking about 20 minutes, fill a 4-quart Dutch oven about half full of water. Add 1/2 teaspoon salt if desired. Cover with lid; heat over high heat until the water is boiling rapidly. Add the spaghetti. Heat to boiling again. Boil uncovered 8 to 10 minutes, stirring frequently, until tender but not mushy.
Place a strainer or colander in the sink. Pour the spaghetti in the strainer to drain. Serve with the tomato sauce.