Heat oil in a medium sized pot over medium heat. Sautè the onions for about 90 seconds, or until tender. Add the garlic, cooking another 1 minute. Stir in the rhubarb, chipotle and tomato paste. Add the paprika and season everything generously with salt and pepper. Then pour in the ketchup, water, vinegar, molasses, mustard, and Worcestershire sauce, stirring until combined. When the mixture begins to bubble, lower the heat to a simmer and cook for 20-30 minutes, stirring often, until the rhubarb is very tender and the sauce has reduced a bit. Remove the sauce from the heat and allow to cool at least 10 minutes. Transfer the sauce to a food processor and process until smooth. The sauce will be thick. Water can be added as needed for desired consistency. Use to baste chicken, ribs, pork chops, or seafood on the grill, make pulled pork in the crockpot, or as a sauce for barbecue chicken pizza.
*This sauce thickens and becomes more viscous as it cools. It may need to be thinned with water if refrigerated.