Prepare the Savory Tart Dough and Béchamel sauce in advance. Chill the dough for at least one hour. Preheat the oven to 400F.
Heat a large skillet over medium high heat & add the olive oil. Add the mushrooms and onions and cook for about 10 minutes, stirring frequently. Add the thyme leaves and continue to cook until almost all the liquid has evaporated and the bottom of the skillet is dry. Add salt & pepper to taste, remove from the heat and set aside.
Roll each disc of dough into circles about 10 inches in diameter. The crust should be about 1/8″ thick. Place 1/3 of the filling in the middle of each circle. Leave a border of about 2 inches around the outside of the crust.
Spoon half of the bechamel sauce over the mushrooms. Top with half of the remaining mushroom mixture and neatly fold the edges of the crust over into loose “pleats.” Lightly brush the pleated edges of the dough with the eggwash and sprinkle each tart with 1 Tbsp coarsely grated cheese and 1 tsp coarse salt.
Place the tarts on a lightly oiled baking sheet. Bake for about 20 to 25 minutes or until each galette is golden brown. Remove from the oven, allow to cool slightly and garnish with a sprig of fresh thyme before slicing and serving.
Béchamel sauce
1 Tbsp butter
2 Tbsp flour
1 cup milk
1 egg, beaten until frothy
4 oz coarsely grated Swiss cheese, divided
Salt & pepper to taste
Melt the butter over medium low heat in a saucepan. Whisk in the flour until completely incorporated. Continue whisking over medium low while the butter & flour mixture bubbles, about 3 minutes. This is known as a roux (pronounced “roo”).
Heat the milk until hot, but not boiling. Pour the hot milk, a little at a time, into the roux while whisking continually. The mixture will quickly begin to thicken. Reduce the heat to low and continue to whisk until the sauce is thickened and smooth. Mix in the grated cheese (reserving 2 Tbsp for later use), bit by bit, until completely incorporated. Season with salt & pepper to taste and remove from the heat.
Whisk a little of the hot sauce (a couple of tablespoons) into the beaten egg to temper it. Then whisk the egg mixture into the sauce and set aside.
Savory Tart Dough
2-1/2 cups all-purpose flour
1 tsp salt
1/4 tsp sugar
8 oz unsalted butter, cut into 1/2-inch cubes and chilled
about 2/3 cup ice water
In a large bowl, mix together the flour, sugar and salt. Place into a food processor with the chilled butter. Pulse until the butter is evenly distributed but still in pea-sized pieces. Add the ice water all at once to the flour and butter. Pulse again until the dough just comes together into a mass do not overmix. Using your hands, divide the dough evenly in half and mold into slightly flattened disks. Wrap the disks in plastic and refrigerate for at least 1 hour.