Add flaxseeds to a bowl. Pour tomato mixture over flaxseeds. Let sit for at least 30 minutes to get gelatinous. Add onion and stir to combine.
Spread mixture onto a Teflon lined dehydrator sheet. You can spread it out as one sheet and score it later or spoon individual spoonfuls for individual sized crackers. Individual crackers will dehydrate faster. Dehydrate for 36-72 hours at 110°F.
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Notes
If you want to score the crackers, do so after they have been dehydrating for a while so that the score lines hold their shape. Scoring the crackers will make it easier to break apart later, but it isn't necessary. You can also cut the crackers with a sharp knife.These raw flaxseed crackers will last several weeks in an airtight container at room temperature.