In a large pot over medium heat, combine the water, onion, carrot, garlic, thyme, paprika, mustard, salt, and pepper. Stir, cover, and cook for 6 to 8 minutes, stirring occasionally, until the onion starts to soften. Add the lentils, stir, then add the vegetable stock, water, tomato paste, and bay leaf. Increase the heat to bring to a boil, then lower the heat to medium-low, cover, and cook for 30 to 35 minutes, or longer if needed, until the lentils are fully cooked through. Remove the bay leaf. Add the lemon juice, if using, stir, and serve, sprinkling with chopped chives, if desired. Serves 4-5.