Mix all your ingredients, except the water, in a big bowl, and knead well until it comes together and there are no lumps of butter or dry floury bits.
Slowly add the water. The goal is to have turn it into a 'shaggy' dough that holds together and is easily rolled out. You may not need all the water, but I tend to use it all.
After about 3 minutes it should be a fairly smooth dough. Scrape your bowl down, cover it with a dish towel and leave for at least half an hour, but it can be as long as all day. You don't need it to ferment, you're just letting it rest so the dough relaxes and is easier to roll.
Heat your fan oven to 160°C (180°C if you don't have a fan).
Roughly quarter your dough into equal pieces. Put plenty of flour on a worktop or large chopping board, and with a rolling pin, roll it out one piece at a time until it's thicker than paper, perhaps as thick as craft card (thinner than the corrugated packing stuff). While rolling, have your baking tray ready next to you so you avoid making it so big it won't fit on the tray.
Carefully transfer your sheet to your baking tray, and gently brush with water* and gently pat onto the surface your choice of seeds and sea salt.
Place into your preheated oven for about 18 minutes. These crisp as they cool, so don't be tempted to overbake them. Their colour is a good indicator of whether they're done, so look for a lovely golden brown.
Repeat with the remaining dough if you are baking them all in one session.
They look quite impressive whole, so I prefer to serve them like. People can then snap off pieces of the size they want.