Place half the black beans in a medium bowl and using the back of a spoon or a potato masher, mash the black beans until they are mostly broken down. Add the remaining whole black beans, diced mushrooms, onions, baby spinach, 3 egg whites, panko, garlic, cumin, chili powder, salt, and pepper. Stir until everything is combined, then let the mixture sit for 5 minutes.
Start forming the burger patties. Make 6-8 burger patties, depending on how big you would like them, make sure the sides are nice and smooth. If the patties are not holding together well, add the last egg white and mix before forming the patties.
Heat a skillet over medium heat with oil. When the oil is warm, place the patties in the skillet and allow each side to cook for 4-5 minutes. You can also spray the patties with cooking spray if you do not want to use a lot of oil, but keep in mind you'll need to be extra gentle. Flip gently and allow to cook on the second side for 4-5 minutes. You can add a slice of cheese at this point if you'd like.
Serve the patties on burger buns with chipotle mayo and your favorite toppings, in pitas with hummus, or on a salad drizzled with chipotle mayo as dressing.