1 Scrub potatoes and cook in a pot of boiling salted water until knife-tender, 20-30 minutes, depending on size.
2 Meanwhile, rinse marjoram and shake dry. Pluck leaves, put some aside and finely chop the rest.
3 Halve leek lengthwise, rinse well, drain and cut white-part into 1 cm (approximately 1/4-inch) slices. (Reserve remaining look for another use.)
4 Rinse apple, wipe dry and cut around the core into slices (about 1 cm or approximately 1/4 inch) thick. Finely dice the apple slices.
5 Heat oil in a non-stick pan. Add leek, cover and cook over low heat until softened, about 6 minutes.
6 Add diced apple and chopped marjoram and sauté for 1 minute. Season with salt and pepper.
7 Drain the potatoes, rinse under cold water, peel while still warm and pass through a potato ricer into a bowl.
8 Mix apple and leek well, season with salt and pepper and let cool. Divide over slices of bread, sprinkle with reserved oregano leaves and serve.