1 Mix quark and mineral water in a bowl until smooth.
2 Rinse and drain the tomatoes, cut into quarters, cut out stem ends and remove the seeds.
3 Finely chop tomato pulp and stir into the quark. Season with salt and pepper. Peel zucchini and potatoes.
4 Grate zucchini and potatoes on a box grater. Squeeze portions firmly, collecting liquid in a bowl. Let stand until the potato starch settles to the bottom, 1-2 minutes. Pour off liquid and mix starch with the grated vegetables.
5 Heat oil in a large non-stick pan. Season vegetables with salt and pepper. Use a tablespoon to scoop grated vegetables into pan, making 6 small rosti. Slghtly flatten in the pan with a spatula. Cook over medium heat until crispy, about 3 minutes on each side.
6 Meanwhile, rinse chives, shake dry and cut into thin rings. Serve rosti with quark mixture and chives.