In a large bowl, whisk together flour, salt, garlic powder, and rosemary until combined.

Add olive oil and water. Stir until a smooth, pliable dough comes together. It should not be sticky, but it will be slightly oily. If it feels too dry, add water 15g / 1 tbsp at a time. If it feels tacky, add a small pinch of flour.

Divide the dough in half. Working with one half at a time, place the dough between two sheets of parchment paper and roll out as thin as possible — you're aiming for about 1/16 of an inch. The thinner you roll, the crispier the cracker.

Peel off the top sheet of parchment. Using a pizza cutter or sharp knife, trim the uneven edges of the dough to create a clean rectangle. Add those trimmed pieces to the second half of dough and set aside.

Transfer the parchment with the trimmed, rolled dough directly onto a baking sheet. Score into roughly 1½-inch squares or rectangles. You do not need to separate them — they will snap apart after baking.

Repeat steps 4–6 with the second half of dough (which now includes the trimmed scraps). Roll, trim, transfer to a second baking sheet, and score.

Lightly brush the surface of both sheets with water and sprinkle with a pinch of flaky sea salt.

Place both baking sheets in the oven at the same time. Bake for 15–18 minutes, swapping the pans halfway through to ensure even baking, until the crackers are golden and firm to the touch. Watch closely after the 14-minute mark — they go from golden to burnt quickly.

Remove from oven and let cool completely on the pans. They will continue to crisp up as they cool. Once fully cooled, snap apart along the scored lines and store in an airtight container at room temperature for up to 1 week.