Heat the oil in a large saucepan over medium heat. Saut� the onion, celery, and carrot for 5 minutes until softened. Add the garlic, mustard seed, and fennel seed, and saut� for 1 minute.
Bring the stock, water, lentils, thyme, and bay leaf to a boil over medium-high heat. Reduce the heat, partially cover the mixture, and simmer for 25-30 minutes until the lentils are tender. Drain, reserving the cooking liquid. Place the drained lentils into a large bowl, and discard the bay leaf.
To make the vinaigrette, mix the vinegar, mustard, and 1/4 cup of the reserved cooking liquid in a small bowl. Add the extra-virgin olive oil and whisk well.
Add the vinaigrette, parsley, salt, and pepper to the lentils. To coat evenly, gently toss the mixture. Serve warm.
