Preheat the oven to 175°C fan. Put oats, flour, cheddar cheese, parmesan, paprika, pepper, cayenne, salt and poppy seeds in the bowl of a food processor with a metal blade. Pulse a few seconds to mix.
Add the butter to the bowl and pulse until the mixture resembles coarse sand. Add the cold water and pulse again until the mixture starts to come together and form a dough. Don’t overdo it.
Put dough on the kitchen counter and gather to a round disc. Divide into two parts. Lightly sprinkle with flour and roll each piece into a rectangle (or whatever shape you like, it doesn't matter), 2-3 millimeters thick.
With a pizza cutter (or better with a wavy cutter) cut strips and then squares of about 4-5 cm. With a wooden skewer, “drill” 5 holes on each cracker (not with the pointed side).
Line two or three oven trays with non-stick paper. Place crackers on the baking pans, about 1 cm from each other. Sprinkle very lightly with a little sea salt. Bake for 18-20 minutes until golden.
Cool on a wire rack. Resist the temptation to eat them right away as the crackers may be soft right out of the oven. But they will become very crunchy once cool.
You will make about 80 crackers. Store them in a cookie jar.