Preheat your oven to 250 F (120 C). Line a cookie tray with baking paper.
Whisk together the 1/2 egg white and vanilla extract with a fork until slightly frothy. Add the almonds and pecans and stir until coated. Pile onto the lined tray (leave behind any excess liquid in the bowl). Pour the maple syrup over the pile of nuts then sprinkle on the sugar, flax seeds and salt. Use your hands to toss the nuts with these ingredients, then spread out in a single layer over the cookie tray.
Bake for 15 minutes, remove from the oven and stir around. Then put back into the oven and increase the heat to 350 F (180 C) - bake for a further 8-10 minutes until the nuts are toasted. Let cool then store in an airtight container.