In a large mixing bowl, combine the mature sourdough starter, semolina flour, olive oil, and fine sea salt. Mix until the ingredients begin to come together.
Gradually add the warm water and filtered or spring water. Use your hands to knead the mixture until a smooth dough forms. If the dough feels too dry, add a little more water, one teaspoon at a time.
Cover the bowl with a damp cloth or plastic wrap and let the dough rest for about 30 minutes. This allows the semolina to hydrate fully and makes rolling easier.
While the dough is resting, preheat your oven to 375°F (190°C). This ensures that your crackers will bake evenly.
After resting, divide the dough into two portions for easier handling. On a lightly floured surface, roll out one portion of the dough to about 1/8-inch thickness.
Using a pizza cutter or knife, cut the rolled-out dough into squares or rectangles. Transfer the pieces to the prepared baking sheet, leaving a little space between each cracker.
Bake in the preheated oven for 15-20 minutes, or until the crackers are golden brown and crisp. Keep an eye on them to avoid burning.
Once baked, remove the crackers from the oven and let them cool on a wire rack. Enjoy them as a snack, with cheese, or as a part of a charcuterie board!