In a small bowl, combine ground flax and water. Stir to combine; set aside.

Mash black beans with a potato masher or fork in a large bowl until most of the beans are a paste.

Pulse the cashews in a food processor fitted with the S-blade until they’re breadcrumb size, with just a few larger pieces remaining. Add them to the bowl with the beans.

Heat two tablespoons olive oil in a skillet over medium heat.

Add onion; cook for 5 minutes. Add mushrooms; cook for another five to seven minutes, or until lightly browned. Set aside to cool slightly.

Add the flax mixture to the bean mixture, then add the remaining ingredients, including the onion-mushroom mixture. Mix very well.

In the same pan used for the mushrooms and onions, heat the remaining two tablespoons oil over medium heat.

With oiled hands or disposable gloves, shape 1/2 cup of the mixture into 3/4-inch (two-centimeter) thick patties. Place three to four patties in the skillet; cook for four to five minutes, until golden and crispy.

Carefully flip the burgers; cook for an additional four to five minutes. Transfer to a paper-towel-lined plate.

Repeat with remaining burger mixture.