In a 5- to 6-quart pan over high heat, bring broth, lentils, onion, carrots, celery, and thyme to a boil. Reduce heat, cover, and simmer 25 minutes, stirring occasionally. Add sherry and simmer until lentils are tender to bite, 5 to 10 minutes longer. Drain; reserve liquid.
Pour lentil mixture into a bowl and let cool to room temperature, about 40 minutes, stirring occasionally. Meanwhile, return cooking liquid to pan and boil over high heat until reduced to 1/2 cup, about 5 minutes. Let cool.
Stir liquid, chopped red peppers, and green onions into lentil mixture. In a bowl, mix oil and vinegar; stir half the dressing into lentil mixture. Add salad mix to bowl with remaining dressing; mix.
Spoon equal amounts of salad mix onto plates. Top equally with lentil mixture. Add salt and pepper to taste.