Heat the oven to 200°C/400°F/gas mark six. Mix together the oatmeals and oats and spread out on a half sheet baking pan. Bake for about 15 minutes, shaking the tray occasionally, until they start to smell toasted. (I liked the idea of toasting the flours before baking to amp up the flavor, but I didn’t realize at the time the effect it would have on the brittleness of the final product.)
Tip into a mixing bowl and allow to cool slightly, then whisk in the salt and sugar. Stir the butter into the boiling water until melted, then stir this into the oats to make a sticky mixture. If it seems too wet to hold together, add a little more of the medium oatmeal, but it should be quite damp. (The dough was damp, but was it damp enough? I really didn't know then, but probably not.)
Butter the lined baking pan. Dust a work surface with medium oatmeal and put the mixture on there. Pack together well and flatten or roll out with a well-dusted pin until it is about 5mm (about ¼’) thick.
Cut out rounds of the size of your choice, then use a palette knife (or spatula) to carefully lift each one on to the tray, still in the cutter as they will be fragile. Space them out well and re-roll any scraps until all the mixture is used up. (I rolled these as indicated and used a 2-1/2 inch biscuit cutter, and boy, were they fragile…I couldn’t even get them to the tray without crumbling! My theory is that toasting the oat flour dries it out, so it’s really going to absorb the water added.)
Bake for 20 minutes, then very carefully turn them over and bake for 10 more minutes until they feel hard and dry on both sides. (My baking time wasn’t even close to that 20 minutes...it was more like 10 minutes on the first side and 5 minutes on the second. Turning them also proved to be disastrous. I did like that my kitchen smelled nutty as they were baking, though.)
Gently transfer to a wire rack to cool, then store in an airtight tin. (Gently transfer, indeed...only 2 made it without breaking up.)