Preheat the oven to 325°F degrees.
Line gerllthe baking sheet with parchment paper. You may also line the baking sheet with aluminum foil (or simply grease the baking sheet very well with additional oil or butter).
In a medium bowl, pour the sourdough starter discard.
Add the olive oil, 1 tsp. of salt, and 1 tsp. of the dried rosemary to the bowl. Whisk well until the oil is completely incorporated with the discard. You should see no pools of oil. (See video.)
Pour the mixture onto the lined or well-greased baking sheet.
Using a spatula or an offset spatula, spread the discard batter so that it completely covers the sheet pan from corner to corner.
If desired, sprinkle the remaining salt and dried rosemary on top of the discard batter.
Place the sheet pan on the middle rack of the oven and bake for 30 minutes.
Using pot holders, remove the sheet pan from the oven after 30 minutes and place the sheet pan on a heatproof surface.
Using a pizza cutter or sharp knife, score the discard batter into the size of crackers that you desire. This is easy to do if your sheet pan is lined with parchment paper. If you used aluminum foil or an unlined sheet pan, you need to do this VERY LIGHTLY so as to not cut through the aluminum foil or scratch the sheet pan.
Return the sheet pan to the oven and continue to bake the scored sourdough discard until it is golden brown and crisp. This will take an additional 15-30 minutes.
Once the sourdough discard is finished baking, turn off the oven, carefully remove the sheet pan from the oven, and transfer it to a heatproof surface. Allow it to cool completely.
Once cooled completely, break the crackers along the scored lines, transfer the crackers to a plate, and serve.
If you prefer not to score your discard batter, you can simply leave your sheet pan in the oven and not remove it at the 30-minute mark. Allow the discard batter to finish baking for an additional 15-30 minutes. When it is golden brown and crisp, turn off the oven and carefully remove the sheet pan from the oven to a heatproof surface. Allow the large cracker to cool completely, and then break it into smaller rustic-shaped crackers. Transfer the fresh crackers to a plate and serve.
If you want, you can store these sourdough crackers in an airtight container in your pantry. They will stay fresh and crisp for at least two weeks. If you live in an exceptionally humid or damp climate and the crackers become soft, they can easily be re-crisped on a sheet pan in a 200°F oven for approximately 5-10 minutes.