The combination of sweet caramelized onions, earthy mushrooms and rich Gruyère cheese, complemented by the perfect addition of quail eggs encased in a buttery pastry, will tantalize your taste buds and leave you craving more with every bite.
Preheat the oven to 350°F.
In a food processor, add all ingredients for the dough except water. Blend on high speed until the dough reaches a sandy texture. Add 1 tbsp water and mix slowly until the mixture becomes a cohesive dough. If necessary, add more water, 1 tbsp at a time. Remove the dough from the food processor and shape it into a disc. Cover with plastic wrap and place it in the fridge to chill.
While the dough chills, caramelize the onions and cook the mushrooms.
For the caramelized onions, place sliced onions in a saucepan along with butter, olive oil, salt, and pepper. Cook for 30-40 minutes over very low heat, stirring occasionally, until the onions are soft and golden.
For the mushrooms, melt butter and thyme in a medium saucepan over medium heat and sauté them in batches until golden. Place the cooked mushrooms on a plate and continue to cook the next batch.
Remove the dough from the fridge and place it on a floured surface. Divide it into 5 pieces and roll out each piece of dough into rough circles measuring 12cm ( 4 1/2 to 5 inches) in diameter.
Add caramelized onions and cooked mushrooms to the center of the rolled-out dough, leaving a 2cm ( 1 inch) border without filling.
Gently fold the excess pastry over the filling, pleating to hold it in place.
Prepare an egg wash by whisking one quail egg with approximately 1/2 Tbsp of water. Brush the pastry with the quail egg wash and sprinkle flaky salt on the crust (optional).
Place Gruyère cheese on top of the filling, making sure to leave space in the center for the quail egg, and sprinkle with chopped sage leaves.
Bake for 30-35 minutes or until the crust is golden.
Remove the tray from the oven, place a quail egg in the center of each galette, and bake for another 3 minutes. The egg whites should be cooked, and the yolks should be runny.
Cool the galettes on a wire rack for at least 10 minutes, and then they are ready to enjoy!