In a large skillet, heat the oil over medium heat. Add the onions, celery and garlic and sauté for 5 minutes. Stir in the lentils, chili powder, and cumin. Cook for 1 minute. Add broth, cover and cook for 15 minutes or till lentils are tender.
Remove the lids and cook about 10 minutes longer, stirring often till the lentils are thickened. Stir in the salsa.
Wrap the tortillas in foil and heat in oven for 5 minutes. Divide the lentil mixture among the tortillas and top with lettuce, cheese, tomatoes and sour cream.