Heat the oil in a heavy saucepan over medium heat. Cook the onion slowly until very soft and slightly golden, 5 minutes. Add the garlic and cook 30 seconds more. Stir in the tomato paste and increase the heat to medium high. Cook, stirring, until the tomato paste thickens.
Whisk in the water, vinegar, mustard powder, salt, pepper, Splenda Granular, Worcestershire sauce, liquid smoke (if using), and cayenne. Simmer the sauce for 15 to 20 minutes to mellow the flavors. Refrigerate in a covered container until ready to use.