Brush 13x9-inch (3-quart) glass baking dish with 1 teaspoon of the softened butter; sprinkle garlic in bottom of dish. Set aside. Cut off stalks from fennel bulbs to within 1 inch of bulb; reserve 1/4 cup of the feathery leaves and discard stalks. Cut off and discard bottom of bulbs. Cut bulb lengthwise into quarters; remove core. Cut each quarter crosswise into about 1/4-inch-thick slices to make about 4 cups.
Heat oven to 375°F. In 12-inch skillet, melt remaining butter over medium-high heat. Add chicken; sprinkle with salt and pepper. Cook and stir about 4 minutes. Add fennel; cook and stir 4 minutes longer. Cover; remove from heat.
Arrange 1/2 of the potato slices in baking dish. In small bowl, mix Alfredo sauce, milk and 2 tablespoons of the fennel leaves. Pour 1/2 of sauce mixture over potatoes in dish; top with 1 cup Swiss cheese. Spoon chicken mixture evenly over cheese. Top with remaining potatoes; press to make mixture flat. Pour remaining sauce mixture over top. Sprinkle with remaining 1 cup Swiss cheese.
Cover with foil; bake 1 hour. Uncover; sprinkle with Parmesan cheese. Bake 25 to 30 minutes longer or until potatoes are tender when pierced with a fork. Let stand 10 minutes before serving. Garnish with remaining fennel leaves.