Peel and wash the potatoes and cook them in boiling salted water for about 20 minutes. Dice tomatoes. Crumble feta. Clean, wash and halve cherry tomatoes. Wash oregano, shake dry and, except for a stalk for garnishing, strip leaves from the stalks.
Peel and chop the onion. Heat the oil in a pot and fry the onion until transparent. Add the chopped tomatoes, bring to the boil, simmer for about 5 minutes and season with sugar, salt, pepper and oregano, except for a little bit for sprinkling. Refine with cream.
Peel the cucumbers, halve them lengthwise and remove the seeds. Drain potatoes, add butter and milk to the potatoes and mash to mashed potatoes. Mix tomatoes and feta, except for something to sprinkle. Spread the mashed potatoes into the cucumbers. Put the tomato sauce and cherry tomatoes in an ovenproof dish spread with oil. Spread the cucumbers on top and sprinkle with the rest of the feta.
Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 35 minutes. Sprinkle the finished cucumbers with the remaining oregano and garnish with the oregano stalk.