Wash potatoes thoroughly and cook in boiling salted water for about 15 minutes. In the meantime peel onion and cut into rings. Wash the rocket and let it drip off. Wash chives, dab dry and cut into rings.
Set aside some chives. Drain the potatoes, rinse and slice. Mix yoghurt and yoghurt dressing. Season to taste with salt, pepper and sugar. Fold in chives, rocket and potato slices.
Wash the meat, dab dry and cut off the fat edge. Grill the meat and tomato for 1-2 minutes on a hot grill or alternatively in a hot grill pan coated with oil, season with salt and pepper.
Remove the tomato and immediately sprinkle with Emmental cheese. Arrange meat, tomato and potato salad on plates. Sprinkle salad with onion rings and remaining chives.