1. Combine rhubarb, sugar, and a pinch of salt in a medium saucepan and let sit until rhubarb has softened and released some of its liquid, about 30 minutes, stirring occasionally.
2. Bring mixture to a boil over medium heat and cook, stirring occasionally, until rhubarb is broken down, 6 to 8 minutes. Let cool slightly, then transfer to a blender and puree until nearly smooth, 30 to 60 seconds; set aside.
3. Rinse saucepan, then add oil and heat over medium heat until shimmering. Add onion and cook, stirring occasionally, until softened and golden brown, 5 to 7 minutes.
4. Add garlic, chili powder, and cayenne and cook, stirring constantly, 30 seconds. Add rhubarb puree, ketchup, molasses, Worcestershire sauce, 1 tablespoon vinegar, and the mustard and stir to combine. Return to a simmer and cook, stirring occasionally, until mixture is thick and reduced to about 2 1/2 cups, 25 to 30 minutes, adjusting heat as necessary to maintain a medium simmer.
5. Remove from heat and stir in remaining 2 teaspoons vinegar and 1/2 teaspoon salt; let cool to room temperature. Refrigerate in an airtight container for up to two weeks.