Preparation Melt butter with oil in heavy large skillet over medium-high heat. Add onion; sauté until golden, about 5 minutes. Add peppers; sauté until just tender, about 3 minutes. Add olives, mustard and garlic. Stir 1 minute. Remove from heat. Season with salt and pepper. Transfer relish to bowl. Cool. Cover and refrigerate overnight. (Can be made 2 days ahead. Chill.) Bring to room temperature before serving.
Preparation Melt butter with oil in heavy large skillet over medium-high heat. Add onion; sauté until golden, about 5 minutes. Add peppers; sauté until just tender, about 3 minutes. Add olives, mustard and garlic. Stir 1 minute. Remove from heat. Season with salt and pepper. Transfer relish to bowl. Cool. Cover and refrigerate overnight. (Can be made 2 days ahead. Chill.) Bring to room temperature before serving.