For the lentils: In a large saucepan, combine the lentils, bay leaves, garlic, celery, thyme, carrot, fennel, red onion and a good amount of salt. Add water to the pan until it covers the ingredients by about 2 inches. Bring the water to a boil over medium heat, and then reduce the heat to a simmer. Cook until the lentils are soft, 20 to 30 minutes. Ladle off 1 cup of the lentil-cooking water and reserve. Remove all the vegetables and aromatics from the lentils and discard. Drain the lentils.
For the warm salad: Heat a large saute pan with enough olive oil to coat the pan. Add the diced carrots, fennel and onions and sweat the vegetables about 5 minutes. Add in the garlic and saute 1 minute more. Add in the lentils, red wine vinegar and mustard. Pour in a couple tablespoons of the reserved cooking liquid and cook down for a couple minutes. Season with salt and garnish with chopped parsley and fennel fronds.