In another skillet, saute onions and celery in oil.
Prepare the flax "egg" by combining the ground flax with water and allowing to rest while we get everything else together.
Meanwhile, prepare mushrooms by removing stems, wiping clean. I used a food processor to chop them into a fine dice, but I almost preferred using a knife which left it chunkier. No large pieces.
In a large bowl, add the flax mixture along with the remaining mushroom steak ingredients. Combine with a fork until coarse and crumbly. Season with salt and pepper.
At this point the mixture should hold together. If it seems a bit dry, add a Tablespoon of water (or you could add a splash of almond or coconut milk). If it seems too wet, add more breadcrumbs.
Form the mushroom mixture into patties and place into a pre-heated skillet with a bit of oil until dark golden on one side and then repeat on the other side. Remove to paper towels to rest for a few minutes, as they will firm up. ***The better the non-stick skillet, the less oil you will need. ***Alternately, you could bake these but I find they get a bit dry.
Serve topped with the mushroom onion gravy, garlic mashed potatoes with your favorite green vegetable for a side.