In a stand mixer with the paddle attachment, mix the flour, spelt flour, oil, salt, and sourdough starter until combined.
The dough is ready to use right away or you can cover and let proof for 1 to 2 hours at warm room temperature (74-76 degrees).
Preheat the oven to 350°F and line two baking sheets with parchment paper and then move the parchment paper to your working surface to roll the dough directly on the paper.
Lightly flour your working surface then lightly flour the dough before turning the dough onto the working surface.
Divide the dough in half.
Place one half on each piece of parchment paper then using a rolling pin, roll each dough half very thin (about 1/4 inch thick).
Make sure to roll the dough evenly so the crackers bake evenly.
Using a fluted pastry wheel (pizza wheel or chef's knife will also work) and cut into 1 1/2 inch squares then lightly brush water onto the crackers with a pastry brush and sprinkle with Za'atar seasoning and flaky sea salt.
The crackers will puff during baking or you can dock the center of each square (using a fork poke, one hole in the center) if you prefer then thin and crispy.
Transfer the parchment paper back to the baking sheets then bake for 10 minutes.
Rotate the pans and switch racks and bake for 10 more minutes or until the crackers are brown and crispy.
Let the crackers cool completely on the baking sheets.
Store in airtight containers at room temperature (up to 1 week).