Preheat the oven to 350°F (180°C). Have two baking sheets and two sheets of parchment paper ready.
In a large mixing bowl, stir together the unfed starter, olive oil, both flours, and salt until all of the flour is just mixed in.
Dump the dough out onto a work surface.
Knead it by hand for about 30 seconds, or until smooth and pliable, then shape it into a ball.
Sprinkle the top of the dough ball lightly with flour then slice it in half.
Place each half of dough onto a sheet of parchment paper.
Using a rolling pin, roll each piece of dough quite thin, about 1/16-inch (2-mm) thick. The dough will have misshapen edges. This is normal.
Brush the rolled dough lightly with olive oil.
Use a pizza cutter or a sharp knife to slice the crackers into 1 ½-inch (4-cm) squares.
Top the crackers with flaky salt and za’atar (or any other seasonings or small seeds you like).
Transfer the crackers, on the parchment paper, to the two baking sheets.
Place the crackers in the preheated oven and bake them for 22 to 25 minutes, or until the squares are beginning to brown around the edges, rotating the pans halfway through baking.
Move the crackers to a wire rack and cool them on the baking sheets.
Once cool, the crackers can be stored in an airtight container at room temperature for up to a week.
Notes
If you don’t have unfed sourdough discard, you can use bubbly active starter. Weighed is best for this as discard and active starter have very different volumes.