Arrange potatoes in a broiler-safe 11 x 7-inch baking dish coated with cooking spray. Cover with plastic wrap; vent. Microwave at HIGH 10 minutes; uncover carefully.
Melt butter in a medium saucepan over medium heat; swirl to coat. Add thyme and garlic; sauté 3 minutes. Sprinkle with flour; stir in 1/3 cup milk. Cook 1 minute, stirring constantly with a whisk. Stir in remaining 2 2/3 cups milk. Bring mixture to a simmer; cook 4 minutes or until slightly thick, stirring frequently. Remove from heat. Stir in salt, pepper, and Gruyère. Pour sauce over potato mixture; sprinkle with Parmigiano-Reggiano. Bake at 400° for 20 minutes or until potatoes are tender when pierced with a knife.
Turn on broiler (leave dish in oven). Broil gratin 2 minutes or until browned. Sprinkle with chives.
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