In a large saucepan, heat 1 tablespoon of olive oil over medium heat. Add the onion, celery and carrot and saute until the vegetables are softened, about 5 minutes. Add the garlic, mustard seed and fennel seed and saute until the spices are fragrant, about 1 minute.
Add the stock, water, lentils, thyme and bay leaf. Raise the heat to medium high and bring to a boil. Reduce the heat to low, cover partially and simmer until the lentils are tender but still firm, 25 to 30 minutes. Drain the lentils, reserving the cooking liquid. Transfer the lentils to a large bowl and discard the bay leaf.
In a small bowl, combine the vinegar, mustard and 1/4 cup of the reserved cooking liquid. (Discard any remaining liquid or reserve for another use.) Whisk in the remaining olive oil.
Add the vinaigrette, parsley, salt and pepper to the lentils and toss gently to mix and coat evenly. Serve warm.