In the cup of a blender, combine the roasted pepper, egg, lemon juice, remaining 2 tablespoons of mustard, and garlic. Blend on until smooth. Season with salt and pepper. With the blender running, slowly drizzle in the oil until the mixture is slightly thick. Add the buttermilk and continue to blend for 1 minute. Season with salt and pepper. Cover and refrigerate for at least 1 hour. The dressing will keep for up to 2 days in the refrigerator.