1. Preheat oven to 400°F. Line a half sheet pan with parchment paper and set aside. Place flours and salt into the Food Processor Work Bowl and secure the lid. Pulse 3-4 times to evenly mix ingredients.
2. Drop the butter evenly over the mixture and secure the lid. Pulse 6-8 times or until mixture has combined evenly, creating a sand-like consistency. Remove the lid and pour in the water over the mixture evenly.
3. Pulse mixture an additional 12-14 times or until dough begins to come together. Remove to a lightly floured surface, and gently knead until dough just comes together. Shape into a 6" round dough ball and wrap with saran wrap, chilling for 2-4 hours.
4. While dough is chilling, prepare the filling for the galette. Fit the Work Bowl with the Thick Slice Blade and secure the lid. Start the machine and place the leeks through the Small Food Chute, continuing to process until all leeks have been sliced, followed by the mushrooms in the Large Food Chute until all have been processed.
5. Heat a large non-stick skillet over medium-high heat. Add about two tablespoons of reserved sun-dried tomato oil to the pan and heat. Add the leeks, mushrooms, and rosemary, cooking for 10-15 minutes until well cooked and the majority of liquid has evaporated, stirring often. Remove from heat and stir in sun-dried tomatoes. Place mixture on a sheet tray or in a baking pan and refrigerate until well-chilled.
6. Remove the dough and mushroom mixture from the refrigerator. Roll the dough out on a lightly floured surface into a rectangle approximately 14" by 14", 1/4" thick. Trim any excess dough. Gently use the rolling pin to roll the dough onto the pin and transfer to the prepared half sheet tray. Unroll the dough and and spoon the mushroom leek filling into the center of the dough, gently folding the edges in about 1 inch over on top of the filling, until a rim has been created around the galette.
7. Brush the rim of the dough with remaining sun-dried tomato oil and sprinkle with sea salt if desired. Place into preheated oven and bake for 25-35 minutes or until dough is cooked and lightly browned. Cool slightly and serve.