Step 1: Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper.
Step 2: In a medium mixing bowl, whisk together the sourdough discard, 1/4 cup olive oil, and water until well combined.
Step 3: Add the all-purpose flour, 1 teaspoon dried rosemary, and 1/2 teaspoon sea salt to the wet mixture. Mix with a spatula or your hands until a shaggy dough forms and no dry spots remain. Do not overmix.
Step 4: Divide the dough into four equal portions. Take one portion and place it between two sheets of parchment paper. Using a rolling pin, roll the dough out as thinly as possible, aiming for about 1/16 to 1/8 inch thickness. Repeat with the remaining dough portions.
Step 5: Carefully peel off the top layer of parchment paper from each rolled-out dough. Using a pizza cutter or a sharp knife, score the dough into desired cracker shapes (squares or rectangles). Lightly brush the top of the scored dough with a little extra olive oil and sprinkle with additional sea salt and rosemary, if desired.
Step 6: Carefully transfer the bottom parchment paper with the scored dough onto your prepared baking sheets. Bake for 15-20 minutes, rotating the baking sheets halfway through, until the crackers are golden brown and crisp. Watch them closely as they can burn quickly.
Step 7: Once baked, remove the crackers from the oven and let them cool completely on the baking sheets. As they cool, they will become even crispier. Once cooled, break them apart along the scored lines. Store in an airtight container for up to one week.