In medium skillet over medium heat, add the olive oil and, when hot, add the onions and garlic and cook for 2 minutes. Add the cabbage, pepper, salt and vinegar. Mix thoroughly and cook until the cabbage is tender, 3 to 5 minutes. Transfer to a bowl, cover with plastic wrap and refrigerate until cool.
In large salad bowl, add the lettuce and toss with the Blue Cheese Dressing. To make sure you add enough dressing for the salad, pour the dressing once around the outside of the bowl and then toss. Add more only if desired.
Add the cabbage to the lettuce. Toss to combine and adjust the seasoning, as needed.
In a food processor fitted with the bottom blade, or in a blender, add 1 cup blue cheese, 2/3 cups buttermilk, Dijon, vinegar, 1/2 of the lemon juice, shallots, salt and pepper and process for a few seconds until combined and creamy. Taste for seasoning and add the remaining lemon juice, if necessary.
Crumble the remaining blue cheese in a medium glass bowl. Pour the blended dressing over the crumbled cheese and fold together with a rubber spatula. Add the remaining buttermilk to thin out to desired consistency. Refrigerate 1 hour before using. Store unused dressing in the refrigerator for up to 2 weeks. Yield: 2 1/4 cups.