Start by grating or cubing the cold butter and placing it in the fridge while you measure out the other ingredients.

In a large bowl, mix the whole wheat flour, all-purpose flour, and salt together.

Add the cold grated or cubed butter to the flour mixture and quickly combine it with your hands or a pastry cutter until the mixture resembles large beans.

Stream in the ice-cold water while mixing the dough with your hands or a spatula. Mix until the dough comes together in a shaggy mass. Avoid overmixing or waiting for the dough to become wet or sticky.

Form the dough into a ball, wrap it in plastic wrap, and refrigerate for at least one hour, or preferably overnight for the best results.

Preheat the oven to 190°C (375°F).

Roll out the chilled pie dough on a floured surface and carefully transfer it to a pie dish. Gently press it into the dish, trimming any excess dough.

To par-bake the pie shell, line it with foil and fill it with beans or pie weights. Bake for 20 minutes. Then, remove the beans and the foil then brush the crust with beaten egg. Bake for an additional 5 minutes.

In a skillet, sauté the diced onion in butter and oil. Season with salt to taste.

In the same skillet, sauté the chanterelles in plenty of butter and oil until they turn golden. Add the thyme, season with salt and pepper, and continue frying until done. Set aside and let cool.

In a mixing bowl, whisk together the cream and eggs. Season with salt and pepper.

Once the pie shell has cooled, layer the cooked onions on the bottom, followed by about half of the cheese, and then the sautéed chanterelles. Pour the egg and cream mixture over the filling and sprinkle the remaining cheese on top.

Place the quiche in the preheated oven and bake for approximately 45 minutes, or until the filling is set and the top is golden brown.

Allow the quiche to cool completely before serving.