For the caramelized onions: In a large skillet, heat the oil over medium heat. Add the onions, balsamic vinegar, salt and pepper. Cook, stirring occasionally, until the onions are deep golden brown, about 20 minutes. Set aside to cool, about 10 minutes.
For the dressing: In a small bowl, whisk together the red wine vinegar, lemon juice and honey. Slowly whisk in the olive oil until blended. Season with salt and pepper, to taste.
For the salad: Peel and trim the ends from each grapefruit. Using a paring knife, cut along the membrane on both sides of each segment. Free the segments and add them to a large salad bowl. Add the lettuce, fennel, cucumber, scallions, and thyme.
Pour the dressing over the salad and toss until all the ingredients are coated. Arrange the caramelized onions on top and serve.