/4 cup balsamic vinegar, 1/4 cup olive oil, 1 teaspoon salt and 1/2 teaspoon pepper.
Bake for 12 to 15 minutes, until the onions are tender.
Remove from oven and toss with 2 more tablespoons balsamic vinegar and cool to room temperature.
Whisk together the shallots, mustard, red wine vinegar, 1/2 teaspoon salt and 1/2 teaspoon pepper in a small bowl. While whisking, add 3/4 cup olive oil until emulsified.
To assemble, toss enough lettuce for 6 people with dressing, to taste.
Place the lettuce on 6 plates and arrange the onions on top.