In a small saute pan, over medium heat, heat the oil and cook the onion until lightly browned. Lower the heat and cook until caramelized, about 10 more minutes, stirring occasionally. Remove from the heat and set aside to cool.
In a large bowl, combine the sour cream, mayonnaise, blue cheese, and sage. If the dressing is too thick, loosen the dressing by pouring in a little more of the buttermilk. You want the dressing to be pourable. Season the dressing with salt, and pepper, to taste.
Combine the spinach, arugula, frisee, tomato, and bacon in a large bowl and dress gently with a few spoonfuls of blue cheese dressing. Add more dressing to taste, but don't overdress the greens. Season the salad with salt, and pepper, to taste.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.