Measure out 1/2 cup of chopped onion. Save remainder in a tightly sealed container, in the refrigerator, for another use.
Measure out 1/4 cup of chopped green pepper. Save remainder in a tightly sealed container, in the refrigerator, for another use.
Pour oil into a 2-quart saucepan.
Place pan over medium heat.
When oil is hot and shimmering, add prepared onion, garlic and bell pepper. Cook 2 minutes, stirring occasionally.
Add tomatoes, with their liquid, tomato sauce, basil, oregano, 1/4 teaspoon salt, and pepper. Stir to mix well.
Increase heat to high. Bring to a boil.
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, prepare pasta according to package directions. Drain. Set aside.
Taste sauce. Add more salt and pepper, to taste, if needed.
Serve spaghetti topped with the tomato sauce. Garnish as desired.