Combine water, lentils, 1/2 teaspoon salt, and bay leaf in a medium saucepan; bring to a boil. Reduce heat, and simmer 25 minutes. Remove lentils from heat, and set aside.
Heat olive oil in a medium cast-iron or nonstick skillet over medium-high heat. Add the onion, celeriac, parsnip, carrot, and 1 1/2 teaspoons tarragon, and sauté 10 minutes or until browned. Stir in 1/2 teaspoon salt, tomato paste, and garlic; cook mixture 1 minute. Stir in wine, scraping pan to loosen browned bits. Bring to a boil; cover, reduce heat, and simmer 10 minutes or until vegetables are tender. Stir in mustard. Add lentil mixture, and cook 2 minutes. Remove from heat; discard bay leaf, and stir in butter, 1 1/2 teaspoons tarragon, and pepper.