Preheat the oven to 350F/160C (gas mark 4). Put the flaxseeds and water in your mixing bowl and let stand 5 minutes. Add the rest of the ingredients and mix until smooth. It should be the consistency of chunk-light tuna fish salad, thick but spreadable (see video).
Cut a piece of parchment paper to fit a rimmed baking sheet. Spray or oil the top of the paper. Spoon the mixture onto the sheet. Put a piece of waxed paper (or a silicone sheet) on top and use a rolling pin to get the dough as smooth and even a sheet as possible.
Gently peel back the waxed paper and discard.
Using a butter knife, score the dough, making small squares. Use your knife to neaten up any jagged edges.
Bake 20 minutes. Put another piece of parchment paper on top, and a fresh baking sheet face down. Flip the entire thing over, and remove the top pan and the top sheet of parchment. Turn the oven down as low as it will go, and bake until absolutely crisp. This could take 4-8 hours, depending on the thickness of the biscuits, your oven temperature, and the humidity outside. (You need to get all the moisture out of them or they will mold.)
If using a dehydrator, bake for 20 minutes. Let the biscuits cool enough to handle, then break up into chunks large enough to fit your trays. Dehydrate overnight at 135 degrees. (Sometimes they take two days.)
Store in an airtight container.
Notes
For dogs on raw food diets, form the biscuit dough directly onto dehydrator trays lined with non-stick sheets. Score as indicated. Dehydrate at 115F/46C until crisp and dry. For grain-free version, omit the oat flour and use more ground flax seeds, just enough for it to stick together.Required FTC disclosure: The Breville Juice Fountain was sent to me by Tesco Direct as a gift. I was not paid to write this post. My thanks to Tesco Direct and Breville for providing me this lovely juicer.