Place the oil in medium, heavy-bottomed (8-inches wide) pot set over medium heat. Add the onion and garlic and cook until softened, about 5 minutes. Add diced tomatoes, tomato paste, sugar, vinegar, salt, pepper, cumin and clove and bring to very gentle simmer (small bubbles should just break on the surface). Simmer, uncovered, stirring occasionally, for 30 minutes, until the mixture is quite thick. Stir more frequently at the end of cooking to ensure the tomato mixture does not burn on the bottom.
Puree the tomato mixture in a food processor or blender. Transfer to a heatproof glass jar and cool to room temperature. Put the lid on the jar and chill the ketchup several hours before using. Ketchup will keep in the refrigerator two weeks.