Place flour, salt, sugar and rosemary in a bowl, whisk to combine.
Stir in water and oil, stir until fully combined.
Turn dough out onto a lightly floured piece of parchment paper (slightly larger than your baking sheet.)
Roll the dough until it is about 1/8th of an inch thick. you may need to add a bit of flour because the dough is quite sticky. Try to keep the thickness as even as possible.
Trim the edges if you want. The edges tend to be thinner and so they may burn in the oven before the rest of the crackers are done.
Cut the dough into squares using a pizza cutter. Brush with a little water and sprinkle with more sea salt if desired. Use a fork to prick each square a few times.
Transfer the dough, still on the parchment paper, onto a baking sheet and place it in the oven. Immediately reduce heat to 425°F.
Bake for 12–17 minutes, or until crackers are starting to become golden.