In a medium saucepan, heat oil over medium heat. Add onion and cook until softened, about 5 minutes. Season with salt and pepper. Add garlic and tomato paste and cook for another minute. Add tomatoes, light brown sugar, 1 tablespoon apple cider vinegar, mustard, ½ teaspoon chipotle in adobo sauce (if using), and rhubarb. Stir to combine. Bring to a gentle boil. Reduce heat to low and cook for 15 to 20 minutes, stirring occasionally, until sauce has thickened and rhubarb is softened. Taste and adjust as desired, adding in additional vinegar or light brown sugar as needed. Remove from heat and allow to cool to room temperature.