Pre-heat the oven to 375º and line a baking sheet with parchment paper.
Clean the mushrooms and break up large clusters, if any.
In a medium shallow bowl, combine the ground flax with the water. Set aside and let it set for about 5-10 minutes (it will thicken and form a gel).
In a second medium shallow bowl, combine the panko breadcrumbs with the onion powder, garlic powder, paprika and salt. Then, one piece of mushroom at the time, dip and completely cover with the flax seed mixture. Then, transfer in the panko mixture and use your fingers to ensure the mushroom is fully covered with no areas free of breading. Transfer to the prepared baking sheet and keep going with the remaining pieces of mushrooms.
Bake in the oven for 20-22 minutes or until golden brown.