Wash the potatoes thoroughly and cut in half. Brush a baking tray with oil and place the potatoes on top with the cut side down. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for about 45 minutes.
In the meantime, put the eggs in boiling water and boil them for about 10 minutes. Quench eggs under cold water and let them cool down. Peel onion. Wash, halve and core the apple. Cut onion, apple and gherkin into fine cubes.
Wash dill and parsley, dab dry and chop. Peel the eggs and dice them as well. Stir yoghurt, mayonnaise and approx. 3 tablespoons of cucumber water (from the gherkins in the glass) until smooth.
Mix in onion, apple, cucumber, egg and herbs and season with salt and pepper. Peel garlic and chop finely. Wash the rosemary, dab dry, pluck the leaves from the stems and chop coarsely.
Mix the remaining oil, garlic and rosemary. Turn the potatoes halfway through the baking time, brush with the seasoning oil and sprinkle with salt and pepper. Cut the rest of the roast into thin slices (slicing thickness) and arrange on plates.
Add baked potatoes and tartar sauce and serve garnished with parsley, egg and apple slices.