Rinse the lentils well in a sieve under running water and put them with the thyme, garlic, bay leaves, the carrot, water and 1 tsp of salt in a saucepan. Add additional water to cover by 2 inches. Bring to a boil, stirring often.
Decrease the heat to simmer and cook for 15 minutes. Add the diced carrots and cook for an additional 3-4 minutes. Do not overcook, as the lentils should still have a light crunch.
While the lentils are cooking, make the dressing . Whisk together the mustard and vinegar in a bowl. Whisk in the oil to emulsify. Add the shallots and fresh tarragon.
Drain the lentils well, discarding the thyme branches, the garlic, the whole carrot and the bay leaves. Combine the dressing with the warm lentils , mixing well, and let it cool to room temperature. Stir occasionally.
When ready to serve, add the chives and the celery. Add additional vinegar, salt and pepper to taste. Add the goat cheese and sprinkle with coarse French sea salt.