Heat oil in heavy medium saucepan over medium-low heat. Add onions and sage. Cover and cook until onions are deep golden brown, stirring occasionally, about 20 minutes. Whisk together mayonnaise and sour cream in a medium bowl to blend. Once they are done cooking, stir in the caramelized onions, then salt and pepper. Cover dip and refrigerate until flavors blend, about 1 hour.