In a large bowl, combine the rolled oats, ground flaxseeds, and psyllium husks. Stir well.
Pour in the water and mix until a thick, evenly combined dough forms. If too dry, add an extra tablespoon of water.
Cover and let it rest for at least 20 minutes (up to 2 hours) to allow the psyllium and flax to bind.
Line a loaf tin with baking paper.
Press the mixture firmly into the tin, smoothing the top. Bake at 180°C (350°F) for 45 minutes.
Carefully flip the loaf upside down onto an oven rack and bake a further 35-45 minutes to crisp the crust all over. You will know the bread is ready when it is firm to the touch. Note: If it still feels soft on top, bake for an extra 5-10 minutes.
Remove from the oven and let the loaf cool fully on a wire rack before slicing.
Slice thinly and store in an airtight container, or freeze in a single layer for the crunchiest toast later.