In a heavy skillet, melt butter over moderately low heat. When foam subsides, add turnips and ptatoes and cook, stirring occasionally, 10 minutes. Add dill, nutmeg, salt and pepper to taste and mix well. Heat briefly. Butter a 15-by-2 1/2-inch oval gratin dish. Transfer turnip mixture to gratin dish and smooth surface with a spatula. (Gratin can be prepared 24 hours ahead to this point and refrigerated.) In a small bowl, whisk together heavy cream and stock, and pour it over turnip mixture in gratin dish. Sprinkle top with bread crumbs and Gruyere cheese. Bake in a preheated 425F oven 20 to 25 minutes, or until top is golden. Serve from gratin dish. Note: If yogurt or low-fat milk are used, bake 5 minutes longer.