Make potato topping: In large saucepan, combine potatoes with enough cold water to cover. Bring to a boil, and cook until tender, about 15 minutes. Drain well.
In large bowl, mash potatoes with 1 cup broth and mustard, and salt and pepper to taste.
Preheat oven to 350°F. Bring small saucepan of lightly salted water to a boil. Add carrots, and cook 2 minutes to blanch. Drain, and set aside.
Coat large skillet with cooking spray, and set over low heat. Add onion, and cook, stirring, until softened, about 2 minutes. Add garlic, and cook 15 seconds.
Stir in lentils, blanched carrots, vegetable broth, tomatoes, tomato paste, oregano and Worchestershire sauce. Mix well. Cook, stirring often, until flavors have blended, about 10 minutes. Season with freshly ground pepper to taste.
Lightly coat 3-qt. gratin dish or shallow casserole with cooking spray. Spoon filling into prepared dish. Top with dollops of mashed potatoes and cheese if desired. (For an elegant look, pipe topping using a pastry bag fitted with star tip.)
Bake until heated through, 30 to 45 minutes. Let stand 10 to 15 minutes before serving.