1. In a 10-inch skillet over medium heat, fry bacon until crisp. With a slotted spoon, transfer bacon to paper towels; crumble and set aside. Pour off all but 2 tablespoons of the drippings. Over low heat, sauté onion in drippings for 3 minutes. Add vinegar, lemon juice, salt and pepper. Simmer for 3 minutes or until onion is tender. Add bacon and remove from heat.
2. Pour dressing over avocados. Toss lightly. Serve on lettuce and garnish with tomato if desired.