Cut Romaine and Iceberg lettuces into 1-inch pieces. In large salad bowl, toss together Romaine and Iceberg lettuces, onion rings, sweet pepper, black olives and Feta cheese. Drizzle with Sherry Wine Vinaigrette. Toss all to coat.
Sherry Wine Vinaigrette: In screw-top jar, combine 1/2 cup olive oil, 1/2 cup salad oil, 1/4 cup sherry or white wine vinegar, 1 finely chopped shallot, 1 teaspoon coarse (kosher) salt and 1/4 teaspoon ground black pepper. Add 1 tablespoon finely snipped fresh tarragon) and 1 tablespoon finely snipped fresh chives. Cover and shake well. Serve immediately or store, covered, in refrigerator up to 1 week. Shake before serving.