Using a large mortar and pestle, combine extra-virgin olive oil, minced garlic, dried rosemary, and dried minced onion. Crush together until rosemary is in small pieces. Allow to set for 10 minutes to infuse flavors. Transfer to a small bowl.
Prepare the Flatbread Dough
In a large mixing bowl, combine the active sourdough starter, warm water, dried milk powder, instant yeast, granulated sugar, kosher salt, and all-purpose flour. Add in 3 tablespoons of the rosemary-garlic oil.
Using a dough hook, mix together until combined and knead for 5-7 minutes. Place in a greased bowl and cover. Allow to rise for 1 hour.
Preheat oven to 450℉ at least 30 minutes prior to baking along with a pizza stone. Use convection oven if that is an option for you.Lay out 2 pieces of parchment paper 10" x 15" and spray with baking oil.
Divide the dough into four pieces and form each into a ball. Place 2 dough balls on each of the parchment papers. Spread each out to form 11" x 5" ovals.
Lightly brush each with remaining rosemary-garlic oil. Allow to rise for 40-50 minutes.
When ready to bake, sprinkle each with sea salt and freshly ground black pepper, as desired.
Bake two at a time on a hot pizza stone for 4-7 minutes until a deep golden brown. Repeat with remaining dough. Serve warm. Makes 4 pieces of flat bread.
Notes
*Use convection setting on your oven if you have that option.