Heat the Leberkäse in the oven on a baking tray lined with baking paper at approx. 160 °.
Cut the cucumber lengthways into thin slices, halve the onions and also cut into thin slices. If you prefer, sweat the onions in a little fat in the pan until they have turned color.
When the Leberkäse is hot, spread some ketchup on each slice, top with onion and cucumber slices and cover with cheese.
Put it back in the oven and bake until the cheese has melted.
Tip: we like to eat mashed potatoes with roasted onions and a cucumber salad with sour cream dressing.