Preheat oven to 325 degrees F (165 degrees C). Grease a 2 1/2-quart baking dish.
Layer potatoes and onion in prepared baking dish.
Melt butter in a saucepan over medium heat; stir in flour until smooth. Gradually add broth, mayonnaise, garlic, salt, parsley, poultry seasoning, and pepper; cook and stir until thick and bubbly, about 2 minutes. Pour mayonnaise mixture over potatoes and onion. Cover baking dish with aluminum foil.
Bake in preheated oven until potatoes are tender, about 1 hour 45 minutes. Remove foil and continue to bake until golden brown, about 15 minutes more. Sprinkle with chives to serve.
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