Combine the all-purpose flour, semolina flour, water, kosher salt, rosemary, and olive oil in the bowl of an electric mixer. Using the dough hook attachment, slowly begin mixing. Once the ingredients are combined, turn the mixer up to medium speed for about 2 minutes until the dough is smooth and springy. Alternatively, mix and knead the dough for 6 minutes by hand.
Note: Different climates affect doughs in different ways, so adjust as needed. Add a bit of all-purpose flour if the dough is too sticky to handle or a bit of water if the dough is not coming together.
Cover the bowl with plastic wrap and allow the dough to sit at room temperature for 1 hour.
Preheat the oven to 400°F.
Line two baking sheets with silicone baking mats or parchment paper. Divide the dough into 6 equal-size balls.
Option 1: Work the dough with a rolling pin.
a. Prepare the dough by picking up a ball in both hands and pulling on either end of the dough, stretching and wiggling it until it is about 5 inches long.
b. Allow the dough to rest briefly (5 min) while stretching the other dough balls, then roll out each stretched piece of dough as thinly as possible (about 1/16 inch thick) with a rolling pin, lifting the dough frequently, rotating it to make sure it isn’t sticking, and adding a little flour when necessary.
Option 2: Use a pasta maker if you have one.
a. Dust one ball of dough with all-purpose flour and flatten it into a disk about 1/2 inch thick. Run the dough through the pasta maker on the widest setting (“1” on most models). Dust the dough lightly with flour again, fold it in half, and feed it through the rollers again; this will help smooth the dough.
b. Adjust the pasta machine to the next setting (slightly thinner than the first). Fold the dough into thirds (as you would a letter) and feed the dough through the rollers with the open-end leading; this will help make the finished cracker ends cleaner.
c. Continue to feed the dough through the machine once on each setting, adjusting the roller to the next-thinner setting each time until the dough is about 1/16 inch (typically the #5 setting).
d. You will have a long, thin sheet of dough about 2 ft long. Cut the dough crosswise into two equal lengths and transfer them to one of the prepared baking sheets. Repeat with another ball of dough and the second baking sheet.
Sprinkle the crackers with the fleur de sel and use a flat-bottomed measuring cup to lightly tamp down into the cracker dough. Bake the crackers for 6 minutes, rotate the trays, and cook for an additional 5 minutes until they are golden brown and crisp. Watch carefully as they go from pale to burnt in seconds. Repeat with the remaining dough.