In 2-quart saucepan, heat 1 tablespoon butter and the onion over medium heat until butter is melted. Sprinkle with potato starch; cook 1 minute. Beat in milk with whisk. Stir occasionally until thickened. Stir in cheeses, mustard, salt and pepper. Stir in cooked pasta; pour into ramekins. Sprinkle with reserved topping.