In a saucepan over medium-high heat, add butter, onions, carrots, and cook until soft, about 15 minutes. Stir in lentils and 5 1⁄2 cups water and fresh herbs. Bring to a boil, reduce heat to medium-low, and simmer until lentils are tender, about 30 minutes. Stir in mustard, season with salt and pepper. Cover and set aside.