Preheat oven to 325º
Line a large baking sheet with parchment paper.
Pulse almonds and hazelnuts in food processor intermittently, until most of the nuts are chopped into large pieces (about ¼ to ½ of the full size.)
Add the pecans. Pulse again, stopping when the pecans are in large pieces.
Add the pumpkin seeds, sunflower seeds, erythritol and flaxseed meal. Pulse just until everything is mixed well. Don't over-process!
Add the egg white to the food processor. Whisk together the melted butter and vanilla, pour evenly into the food processor.
Pulse a few times, mix a littel from the bottom toward the top with a spatula, then pulse a couple times more. Repeat as needed until everything is coated evenly. Again avoid over-processing.
Transfer to baking sheet in a uniform layer, pressing together in a thin rectangle.
Bake for 15-18 minutes, until lightly browned, especially at the edges.
Cool completely before breaking apart into pieces. It will be soft when you remove from the oven, but will crisp up as it cools.
Store in airtight containers in a cupboard with no sunlight up to several weeks. Can be store in refrigerator for longer.