Make the crust:
In the bowl of a stand mixer fitted with the paddle blade, combine the flours, xanthan gum, and sea salt. Add the butter and mix on medium speed until crumbles form. Add the egg and mix again. Add the water gradually and continue mixing until the dough detaches from the bowl and forms a ball. Wrap in plastic wrap and refrigerate for a minimum of 1 hour before using. If it's in the fridge for a few hours or overnight, take out of the fridge at least 30 minutes before using.
Preheat the oven to 400ºF and butter a 10-inch quiche mold (quiche molds have higher sides than regular tart molds).
Make the filling:
To prepare the watercress, start by cutting off the leaves and discarding the longer stems. Make sure to wash it a few times until the water is clear to completely remove the dirt. Dry the cleaned watercress in a salad spinner.
In a sauté pan, heat 2 Tbs. of the oil over medium heat. Add the watercress and sauté for about 3 minutes, stirring, until soft. Transfer to a colander and squeeze out the excess water between your fingers. Set the pan aside to use later. Finely chop the watercress and set aside.
In a pot, heat 3/4 cup of the milk with the bay leaf and a pinch of sea salt and pepper. When just beginning to simmer, add the salmon and poach it for 3 minutes. Remove from the heat, cover, and let infuse for 10 minutes. Remove the salmon and crumble it into a bowl, making sure to discard the bones; set aside.
In a bowl, beat the eggs with the cream and the remaining 1/4 cup milk. Season with sea salt and pepper and add the nutmeg, dill, and sage; set aside.
In the same sauté pan you used for the watercress, heat the remaining 2 Tbs. oil over medium heat. Add the leeks and shallot and cook for about 5 minutes without browning, stirring, until soft. Remove from the heat and stir in the watercress; set aside.
Fill and bake the quiche:
Roll and cut the dough to fit inside the mold. Arrange the dough inside
the mold and using a fork, make small holes at the bottom.
Prebake for 10 minutes. Remove the parchment paper and weights and brush
the crust with the egg white. Return to the oven and bake for 5 more
minutes. Add the vegetables and crumbled salmon to the crust and pour
the egg batter over the top.
Bake the quiche for 30 to 35 minutes (20 minutes if you’re making
individual quiches), or until the top is golden and the egg flan is set.
Let rest for 10 minutes before cutting; serve with a green salad.