Peel the onion and cut into thin slices. Pour with fat into a microwaveable dish. Pre-cook without lid for three minutes at 600 watts. Peel potatoes, cut into slices. Dice ham.
Cut the cucumber into slices as well. Mix the prepared ingredients with the onions. For the sauce, stir cream and cornflour until smooth. Grate cheese into it. Season to taste with salt and pepper and pour into the mould.
Cover and cook for eight minutes at 600 watts. Wash the chives, dab dry and cut into small rolls. Sprinkle the casserole with the chives before serving.