Heat the olive oil in a shallow pan. Add the onion, garlic, bell pepper and tomatoes. Sprinkle in some salt and pepper, and the paprika and oregano. Cook, stirring, over medium heat for about a minute.
Set the heat to low, cover the pan and continue cooking the vegetables for about 10 minutes or until soft.
Pour in the tomato sauce and add a bit of sugar. Stir. Cover the pan and simmer for two to three minutes.
Taste the sauce and adjust the seasonings, if needed.
Add the cooked spaghetti to the pan and toss with the sauce until heated through.
Sprinkle the spaghetti with chopped greens before serving.