In a large skillet over medium-high heat, combine the olive oil, red onion and garlic and cook 2 minutes (do not brown). Add the cabbage, 1 teaspoon salt, 1 tablespoon pepper and the vinegar and mix thoroughly; cook 3 to 5 minutes, until the cabbage is tender. Transfer to a bowl and let cool to room temperature, then refrigerate 30 minutes.
In a large salad bowl, toss the lettuce with the blue cheese dressing. Drain the cabbage and lightly mix with the iceberg and dressing. Add more pepper to taste. Serve immediately.