Place the lentils in a pan with 3 cups water, 1 teaspoons salt, and the bay leaf. Bring to a boil, then lower the heat to a bubbling simmer and cook until the lentils are tender but still hold their texture, about 25 minutes.
Meanwhile, heat the oil in a medium skillet. Add the onion, carrot, and celery. Season with 1/2 teaspoons salt and cook over medium high heat, stirring frequently, until the vegetables are browned, about 10 minutes. Add the garlic and tomato paste, cook for 1 minute more, then add the wine.
Bring to a boil and then lower the heat and simmer, covered, until the liquid is syrupy and the vegetables are tender, about 10 minutes.
Stir in the mustard and add the cooked lentils along with their broth.
Simmer until the sauce is mostly reduced, then stir in the butter and season with pepper. Serve with a sprinkle of freshly chopped parsley.
Divide the lentils among four ramekins or a single gratin dish.
Roll defrosted puff pastry or pie dough to 1/8 inch thick, then cut it just a tiny bit larger than the dish. Set it over the dish and cut a few decorative slashes for the steam to escape.
Bake at 375* F. until the pastry is puffed and golden and the lentils are heated through, about 25 minutes.