For a richer flavor, consider using homemade chicken or vegetable broth. Aged balsamic vinegar provides a more complex and nuanced sweetness. If unavailable, a good-quality regular balsamic vinegar will suffice. Don't overcook the lentils. They should be tender but not mushy. Feel free to experiment with other vegetables, such as diced carrots or celery, added along with the onion. To make ahead, cook the lentils and sauté the onions and garlic separately. Combine and finish the recipe just before serving. For a protein boost, top with crumbled feta cheese or toasted nuts.