Heat the oven to 190 C / 375 F. Line a large sheet pan with parchment paper.

Add the rolled oats, flour, salt, garlic powder, black pepper, and rosemary to a large mixing bowl. Stir to combine.

Add the cold butter cubes. Use a pastry cutter or your fingertips to rub the butter into the dry ingredients until the mixture looks like coarse crumbs with some pea-sized pieces of butter remaining.

Add the finely grated cheddar and toss it through the crumb mixture until distributed.

Add ice water one tablespoon at a time, stirring with a fork after each addition, until the dough just comes together and holds when pressed. Stop as soon as it holds - you may not need all 4 tablespoons.

Turn the dough out onto a lightly floured surface. Press it into a flat disk, wrap in plastic wrap, and refrigerate for 10 minutes.

On a lightly floured surface, roll the dough to about 3 mm thickness. Cut into rectangles or squares roughly 4 x 5 cm, or use a round cutter if you prefer.

Transfer crackers to the prepared sheet pan, leaving a small gap between each one. Prick each cracker three times with a fork.

Bake for 16 to 18 minutes until the edges are golden and the surface looks dry. Check at 15 minutes - oven temps vary.

Remove from the oven and leave the crackers on the pan for 10 minutes to firm up before moving them to a wire rack. They crisp fully as they cool.