Mix potatoes, ham and onion in a greased 2.5 L casserole dish and set aside.
In a 2 L saucepan over medium heat, combine milk, butter, cornstarch, salt and pepper. Continuously stir the mixture with a spatula and cook for 5 minutes or until smooth and thickened.
Pour the sauce over potato mixture. Place the wire rack into the oven on position 2. Place the casserole on the rack. Turn the convection steam oven on by selecting the Convection Humid Mode set to 175°C. Bake for 1 hour or until golden brown and bubbly. Remove the casserole dish from oven and allow it to rest for 15 minutes before serving.