The secret to this bright, crunchy salad-in-a-jar is to toss the cooked lentils with the vinaigrette while they’re still warm. Slideshow: More Lentils
In a medium bowl, whisk the vinegar with the mustard, garlic, 1/2 teaspoon of salt and 1/4 teaspoon of pepper. Drizzle in the olive oil, whisking constantly, until emulsified. You will have 1 1/4 cups; pour 3/4 cup into a large bowl.
In a medium saucepan, cover the lentils with 1 inch of cold water. Add 1 teaspoon of salt, the bay leaf and thyme and bring to a boil. Simmer over moderate heat until the lentils are tender, about 25 minutes. Drain and discard the bay leaf and thyme. Add the lentils to the large bowl and toss to coat. Spread the lentils on a rimmed baking sheet and let cool, about 20 minutes.
In a medium bowl, toss the carrots, celery and onion with the remaining 1/2 cup of vinaigrette to coat. In pint-sized mason jars, layer half of the lentils, then half of the carrot mixture. Repeat the layering with the remaining lentils and carrots. Top each jar with 2 tablespoons of the hazelnuts and 1/2 cup of the herbs.
The layered salads can be refrigerated in closed jars overnight.