Heat olive oil in a medium saucepan over medium heat. Sauté the onions in oil until tender and beginning to brown, 6-8 minutes. Stir in the garlic and sauté for 1 minutes. Add the tomato paste, salt, ground mustard, paprika, cloves, allspice, and crushed red pepper. Stir with a wooden spoon for 1 minute, until fragrant. Add the brown sugar, vinegar, and tomatoes. Simmer over low heat, stirring occasionally, for 45 minutes, until the tomato mixture has thickened.

Remove from heat, and use an immersion blender to blend into a smooth purée. (Or use a regular blender, being careful not to fill the pitcher more than 1/2 full - you may need to do this in batches - and hold a towel over the lid while blending.) Place a fine-mesh strainer over a bowl, and strain ketchup to remove any seeds or large bits of tomato skin.

Taste, and adjust seasoning if needed. Pour into glass jars, and store covered in the refrigerator for up to 30 days.