Bring water to a boil in a small saucepan, then add lentils. Reduce heat to low and simmer the lentils, covered, for 30 minutes or until tender. Drain and rinse with cold water.
In a large mixing bowl, whisk together balsamic vinegar, lemon juice, honey, and thyme. Add walnut or olive oil, and whisk until combined. Add lentils, toasted walnuts, grapes, celery, green onions and feta, and stir until salad is well coated with dressing.
Season with salt and pepper to taste, then serve with some crusty bread and a glass of wine. Leftovers will keep for up to 3 days in the refrigerator.
