Wash the potatoes thoroughly, grate them dry and place them on one half of a baking tray lined with baking paper. Sprinkle potatoes with approx. 2 tablespoons of olive oil, sprinkle with sea salt and cook in a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: level 2) for 1 1/4-1 1/2 hours.
Wash the meat and cut into walnut-sized pieces. Peel onions and cut into slices. Put meat and onions alternately on wooden skewers, season with gyros spice and salt. Heat 2 tablespoons of oil in a pan.
Brown the skewers in it in portions all around for 3-4 minutes. Remove the finished skewers and let them drain. Place the skewers on the baking tray in the oven about 10 minutes before the end of the cooking time of the baked potatoes.
Clean and wash the salad, pluck into bite-sized pieces and drain. Wash the cucumber and tomatoes, dab dry, cut or slice them and put them in a bowl with the lettuce. Peel the vegetable onion and cut into rings, also add to the salad.
Mix mustard and vinegar. Season with salt, pepper and sugar. Add 3 tablespoons of oil. Mix the salad carefully with the vinaigrette. Take potatoes out of the oven and cut them into the middle, spread tzatziki on top and sprinkle with pepper.
Remove the shashlik from the oven and arrange on plates with potatoes and salad. Garnish with parsley.