Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

In a small bowl, combine ground flaxseed and water; let sit for 5 minutes to thicken.

In a large bowl, cream together the softened vegan butter, brown sugar, and granulated sugar until light and fluffy.

Add the thickened flax egg and vanilla extract to the butter mixture and beat until well combined.

In a separate bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the oats, vegan chocolate chips, crushed pretzels, and walnuts.

Scoop rounded tablespoons of dough onto the prepared baking sheets, spaced 2 inches apart.

Bake for 10 to 12 minutes, or until the edges are lightly browned.

Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.