Heat oven to 170C (340F).
Place all the seeds (you can break the larger seeds down a bit first in a food processor, if you like) and first measure of salt in a large bowl. Pour in water and mix to combine (don’t worry – it will be sopping wet, but this will all be absorbed!). Leave for 15 minutes to allow the Chia and Flaxseeds to bind everything together.
Tip mixture onto a baking tray lined with baking paper (I suggest you lightly spray the baking paper with vegetable oil first to avoid sticking) and lightly press the mixture down with a spatula.
Lay another piece of greased baking paper over top (greased side down) and then, using a rolling pin, roll over the mixture gently and evenly until it is about 3-4mm in height.
Peel the top layer of baking paper off. Sprinkle the mixture with flaky sea salt, then bake for 30 minutes.
After 30 minutes, remove the tray from the oven and, using a sharp knife, slice into crackers.
Now, return the crackers to the oven and cook for a further 20-30 minutes until crisp and just turning golden.
Remove to a rack to cool, then store in an airtight container.