Cook pasta according to package directions but reduce cooking time by 2 min. Drain.
Meanwhile, heat half the butter in the same saucepan. Add onion. Cook, stirring for 5 to 8 min, or until tender. Sprinkle evenly with flour. Cook for 2 min. Slowly whisk in milk until smooth. Whisking constantly, bring to a boil. Cook, stirring frequently, for 10 to 12 min or until sauce thickens slightly. Stir in cheese and cooked macaroni
Season chicken with salt and pepper. Heat remaining butter in skillet over medium heat. Cook chicken for 10 to 12 min or until browned and cooked through.
Divide pasta mixture evenly between 4 plates. Top each with chicken and garnish with chives.