In a medium saucepan over medium heat, warm the oil. Add the onion and sauté until softened, but not browned, about 7 minutes.
Add the rhubarb, sugar, vinegar, adobo, and Worcestershire sauce and stir to combine.
Cook over medium heat, stirring frequently, until the rhubarb is softened and the mixture is saucy, about 7 minutes.
Transfer the sauce to a blender and blitz until smooth. Season to taste with salt and pepper. Add a pinch of cayenne, if desired.
Store in a sealed jar or container in the fridge for up to 2 weeks.
Notes
Consistency--If your sauce is too thick after blending, drizzle in water, 1 tablespoon at a time until you reach the desired consistency. If it's too thin, return it to the saucepan and simmer until it thickens.
Dietary--This recipe is suitable for gluten-free diets.