In a large saucepan, heat the olive oil over medium heat. Add the rhubarb, onion, and garlic and cook until softened, 4 to 6 minutes. Add the tomato puree, water, molasses, vinegar, brown sugar, salt, chili powder, red pepper flakes, black pepper, ground mustard, and nutmeg and stir to combine using a spatula. Bring the mixture to a boil, then reduce the heat to low to maintain a simmer and cook for 30 to 40 minutes, until thickened and reduced by almost half to about 4 cups. Stir in the butter and allow it to melt completely. Remove the sauce from the heat and allow to cool for at least 10 minutes.
Using a ladle, transfer the sauce to a blender or place an im- mersion blender in the saucepan and blend until smooth, scraping down the sides as needed. Allow the sauce to cool completely. Store in an airtight container in the refriger- ator for up to 1 week
