Preheat oven to 450º. Melt butter in a large skillet over medium-high heat; add cauliflower, and sauté 4 minutes. Add fennel slices, and sauté 4 to 5 minutes or until tender. Remove from heat. Sprinkle with flour, salt, and nutmeg; add fennel fronds, and lightly toss together.
Spoon into a 2-quart gratin or baking dish. Pour cream over mixture. Sprinkle with remaining ingredients.
Bake at 450º for 20 minutes or until top is browned and crispy.