In a large mixing bowl, combine the unbleached all purpose flour, kosher salt, ground black pepper, onion powder, and nutritional yeast. Use a whisk to combine all of the dry ingredients.

Make a well in the center of the dry ingredients, and pour in the sourdough starter. Next, pour in the melted butter as well.

Use a wooden spoon to combine all of the ingredients until a shaggy, slightly sticky dough forms. Try not to over mix.

Lightly sprinkle the top of the dough with a bit of flour, and use your hands to gently knead the dough into a round smooth ball. This should only take about 4-5 turns of the dough.

Place the dough in a clean small bowl, and cover with a plate or clean towel. You can also wrap the dough in plastic wrap. Refrigerate the dough for at least 1 hour, or up to 48 hours.

Preheat the oven to 400°F.

Remove the dough from the fridge, and let soften slightly. Place a piece of parchment paper on your work surface. Generously dust the parchment with flour, and place the dough on top of the parchment. Dust the top of the dough with flour as well. Roll the dough out to the edges of the parchment sheet, or until it is about 1/16 of an inch thick.

Slide the parchment on top of the baking sheet. Use a pizza wheel the slice the crackers into your desired shape and size - make squares, rectangles, diamonds, circles, or long thin batons.

Drizzle the tops of the unbaked crackers with the olive oil, and sprinkle with flakey sea salt, or your favorite toppings (see notes below).

Bake for 20 minutes, or until golden brown. Bake for an extra 5 minutes for a delicious deeper toasted, nutty flavor.

Allow the crackers to cool completely at room temperature. Then place the cooled crackers in an airtight container, and store at room temperature for up to 4 days.