In a large saucepan, combine the chopped rhubarb, ketchup, brown sugar, apple cider vinegar, and honey.
Add the finely chopped onion, minced garlic, Worcestershire sauce, smoked paprika, mustard powder, salt, black pepper, and cayenne pepper (if using).
Bring the mixture to a boil over medium-high heat, then reduce the heat to low and simmer for about 30 minutes, stirring occasionally.
Once the rhubarb is soft and the sauce has thickened, remove it from the heat and let it cool slightly.
Blend the sauce with an immersion blender or in a regular blender until smooth.
Allow the sauce to cool completely before transferring it to a jar or airtight container.
Store the sauce in the refrigerator for up to two weeks. Enjoy it with your favorite grilled meats or as a unique condiment for sandwiches and burgers.