In your stand mixer with a dough hook attachment, mix together sourdough starter, water, milk, oil, sugar and garlic salt.
Add all purpose flour. (You can also use whole wheat flour or spelt flour)
Knead 4 to 6 minutes. Dough should be soft, stretchy and only slightly sticky.
Place in a greased bowl.
Cover with a tea towel and let rise for 4 hours.
After 4 hours, divide the dough into 6 equal pieces.
Roll flat with a rolling pin. About 1/4 inch thick. It doesn't need to be a perfect circle.
Use a bench scraper to loosen them around the edges if needed.
Preheat your cast iron skillet over medium-high heat.
You can add a little olive oil or melted butter to the pan or if it is well seasoned, you don't need to.
Cook the flatbread for a few minutes until it starts to brown a bit. You want to see bubbles start to form. Bubbles are a good thing! Flip and cook the other side until browned.
Slice with a pizza cutter and serve, or enjoy it brushed with warm garlic butter and fresh herbs!