Preheat oven to 400°. Place potatoes and water in a large microwave-safe bowl; microwave, covered, on high until almost tender, 12-14 minutes., In a 6-qt. stockpot, heat butter over medium-high heat; saute onion and garlic until tender, 5-7 minutes. Stir in flour and seasonings until blended; gradually stir in broth and cream. Bring to a boil, stirring occasionally; cook and stir until slightly thickened, 2-3 minutes. Stir in cheeses until melted., Drain potatoes; add to sauce, stirring gently. Transfer to a greased 13x9-in. baking dish. Bake, uncovered, until lightly browned, about 20 minutes. If desired, sprinkle with chives.