Grease a 9×9″ square baking pan with shortening; set aside.
Combine the psyllium husk and the water in a large bowl or the bowl of a stand mixer. Let it thicken and gel up for about 5 minutes. Add in the sourdough starter and mix to combine.
Add in the starch, flour, and salt. Mix until a smooth dough forms. If the dough seems sticky, let it rest for a few more minutes to let the psyllium and flours absorb.
Press into your prepared pan, or press it into a similarly sized shape on a baking sheet.
Let rise for 3 to 5 hours at warm room temperature, or for up to a day in the refrigerator. The dough will feel lighter and puffy when it’s ready.
An hour before you’re ready to bake, preheat the oven to 425ºF.
When it’s fully risen, drizzle the dough with a healthy glug of olive oil and use your fingers to press dimples into the dough.
Add toppings if desired, or just sprinkle with flaky sea salt.
Bake for 35 to 40 minutes, or until golden brown and baked through.
Let cool for 30 minutes and then remove from the pan and allow to cool completely on a wire baking rack (this keeps the bottom from getting soggy).
Store at room temperature for two to three days, in the fridge for up to a week, or slice and freeze in an airtight container for up to six months.
