Preheat oven to 350°. In a large saucepan, melt 2 tablespoons butter. Stir in flour, salt, nutmeg and pepper until smooth; gradually add milk. Bring to a boil; cook and stir until thickened, about 2 minutes. Remove from heat. Add cheeses; stir until melted. Stir in potatoes., Spoon half the potato mixture into a greased 2-qt. baking dish. Top with broccoli and remaining potato mixture. Bake, covered, 35 minutes., Melt remaining butter; toss with bread crumbs. Sprinkle over casserole. Bake, covered, until heated through and topping is golden, 15-20 minutes.