A Swiss Chasselas, with its moderate acidity and clean, mineral-driven palate is an ideal choice for this tart. The delicate fruit notes provide a nice contrast to the mushroom and underscore the cranberry.
Add flour, butter, a pinch of salt, and a part of the eggs to a standing mixer. Knead until a smooth dough forms.
Place dough in plastic wrap, transfer to refrigerator, and let rest for approx. 30 min.
In the meantime, finely dice onion. Finely slice green onions and chives. Roughly chop cranberries. Clean and cube mushrooms.
Preheat oven to 180°C/350°F. Heat some vegetable oil in a large frying pan and sauté onions until translucent. Add mushrooms and sauté for approx. 8 - 10 min. until any liquid has evaporated. Add cranberries and green onions to mixture, stir, and set aside.
Flour working surface and roll out dough. Transfer to a greased tart pan. Carefully place a sheet of parchment paper on the dough and fill with pie weights or dried beans. Blind bake in preheated oven at 180°C/350°F for approx. 8 – 10 min. Set aside.
Meanwhile, mix crème fraîche, whipping cream, chives and remaining eggs in a large bowl. Season with salt, pepper, and grated nutmeg.
Remove pie weights and parchment paper and fill dough with mushroom mixture. Pour sour cream mixture over mushrooms. Bake in preheated oven at 180°C/350°F for approx. 35 - 40 min.