1. Preheat oven to 400°. Melt butter in a heavy saucepan over medium heat. Add shallot; sauté 2 to 3 minutes or until tender. Add garlic, and sauté 1 minute.
2. Whisk in flour; cook, whisking constantly, 1 minute. Gradually whisk in half-and-half; cook, whisking constantly, 3 to 4 minutes or until thickened and bubbly. Remove from heat. Whisk in cheese until melted and smooth. Stir in salt and next 2 ingredients.
3. Layer potato and fennel slices alternately in a lightly greased, broiler-safe ceramic 2-qt. casserole dish. Spread cheese sauce over layers. Cover with aluminum foil.
4. Bake at 400° for 50 minutes or until potatoes are tender. Remove from oven. Increase oven temperature to broil with oven rack 5 inches from heat. Uncover dish, and broil 2 to 4 minutes or until golden brown. Garnish, if desired.