Bake half a 28-ounce bag frozen potato puffs in a 2-quart baking dish at 425˚ until crisp, 30 minutes. Meanwhile, cook 1 tablespoon each butter and flour in a small saucepan over medium heat, whisking, until melted. Whisk in 1 1/4 cups milk, 1 small grated garlic clove, 1/2 teaspoon chopped thyme, 1/4 teaspoon kosher salt and a pinch of cayenne. Simmer until thickened, 10 minutes. Off the heat, stir in 1/2 cup grated gruyère. Pour over the potato puffs. Broil until browned, 3 to 4 minutes. Top with chopped chives.