Place the frozen Rösti corners on a baking tray. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes. Turn after 8 minutes. In the meantime, peel and halve the avocado for the topping and remove the core.
Cut the flesh into half rings and sprinkle with lemon juice. Then spread on the Rösti corners. Place 1 slice of ham and some crème fraîche on each. Serve sprinkled with pepper.