A simple but delicious cake recipe:
Simmer rhubarb with the water on low heat for 5 min with the lid closed, remove a few for garnish, cut in half lengthwise, set aside.
Continue to cook the rest open without stirring until all the liquid is reduced. Crush rhubarb, cool. Mix curd with cornflour and egg yolk. Spread some Sbrinz evenly on the pastry base, mix the rest with Gruyère and rhubarb puree into the curd mixture, season. Whip the egg whites with the salt until stiff, fold them exactly into the mixture, spread it evenly on the dough base, spread it smoothly, spread rhubarb pieces set aside evenly on top.
Bake for about 50 minutes on the bottom rack of the oven heated to 200 °C. Stand for about 5 min. in the turned off, slightly open oven, maybe garnish.