sweet potato galette - Mallory Oniki

Add the all-purpose flour and whole wheat flour to a bowl (you can also use only all-purpose flour instead, just use 145 grams total). Add salt and mix to combine.
Add cold, cubed butter and toss so all the butter is covered in flour. Break each cube of butter into smaller pieces and then toss in the flour. When all the butter is broken down, add the cold water and mix until just combined. The dough will be very shaggy. Turn the dough out onto your work surface and knead just until the dough comes together.
Shape the dough into a rectangle. Cover with plastic wrap and transfer to the fridge for one hour.

While the dough is chilling, prepare the toppings, starting with the caramelized onions. Thinly slice the yellow onion and add it to a pan with 1 tsp olive oil over medium heat. Stir as the onion begins to cook down. After 5 minutes of cooking, add a splash of water and stir. After another 5 minutes of cooking (while stirring occasionally) add another splash of water and stir. Continue this process until the onions are lightly brown and caramelized. Set aside.
Peel and slice the sweet potato as thinly as you can. It is easiest to use a mandoline if you have one. Transfer the sweet potatoes to a bowl and add olive oil, salt, and pepper. Toss to combine.
Also prepare the herb oil. Add all ingredients to a blender and blend until smooth. Set aside.

Preheat the oven to 400 F. After 1 hour of chill time, remove the crust dough from the fridge. Use a rolling pin to roll out the dough into a rough circle, about 1/4" thick. Add the dijon mustard onto the dough and spread out evenly everywhere except for a 2 inch border on the edge.
Add parmesan cheese and evenly spread on top of the mustard. Add the caramelized onions on top of the cheese. Then, starting from the outside, layer the sweet potatoes, one at a time, slightly overlapping each other, in a circle until the entire galette is covered (other than the 2 inch border).
Fold the border of the crust over towards the middle. Rotate the galette and continue to fold the edges towards the center, until all the edges is folded in.
Crack the egg into a small bowl and whisk to combine. Use a pastry brush to brush the egg wash onto the crust. Transfer to a baking sheet and bake for 30-40 minutes, until the crust is golden brown. Remove from the oven, top with a drizzle of herb oil, slice, and serve immediately!