1. Melt butter in a large saucepan over medium heat. Add onion and garlic; cook 5 minutes or until onion is tender, stirring occasionally. Add rhubarb; cook 3 minutes or until rhubarb is translucent, stirring occasionally. Add 1/2 cup water, sugar, ketchup, vinegar, and chipotle; bring to a boil. Reduce heat; simmer 6 minutes or until rhubarb is tender.
2. Place half of rhubarb mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend rhubarb mixture until smooth. Pour into a large bowl. Repeat procedure with remaining rhubarb mixture.
3. Return rhubarb mixture to saucepan. Bring to a simmer; cook 1 minute or until hot. Stir in mustard and salt.