Drain the liquid from the can of black beans and place it on a parchment-lined baking sheet.
Clean the Portobello mushroom and cut into 1/4-inch thick slices
Line on a parchment-lined baking sheet (nothing overlapping).
Bake beans and mushrooms for 30 minutes, flipping once after 15 minutes.
When the beans and mushrooms have 5 minutes left, prepare a flax egg with the flaxseed meal and water.
Set aside to thicken.
When cooked, the beans should be burst, with the whiter parts of the insides showing, and the mushroom should look dry with no residual liquid on the pan.
After the beans and mushrooms have cooled for 5 minutes, add the flax egg, and all other ingredients to a food processor.
Process and scrape down the sides once or twice until everything is combined, but there is still good texture.
Form the mixture into 4 patties and set aside to rest
Heat a nonstick pan to medium-high heat.
Add the patties to the pan and cook without touching for 3 to 4 minutes.
The patties shouldn’t stick and should be golden brown before flipping.
Flip and cook for an additional 3 to 4 minutes until patties are complete.
Serve hot on a burger bun with fresh toppings of your choice.