1. Drain and rinse the black beans, then pat them completely dry with a paper towel to ensure a firm burger texture.
2. In a medium bowl, mash the black beans with a fork until mostly smooth but still containing some whole bean pieces for texture.
3. Stir in the egg whites, nutritional yeast, crumbled feta, garlic powder, cumin, sea salt, and black pepper until well combined.
4. Divide the mixture into two equal portions and shape them into thick patties.
5. Heat a non-stick skillet over medium heat and lightly coat with a mist of olive oil if needed.
6. Cook the patties for 5-6 minutes per side until they develop a golden-brown crust and are heated through.
7. While the burgers cook, mash the avocado with the Greek yogurt and lime juice in a small bowl until the crema is silky and smooth.
8. Arrange the romaine lettuce leaves on a plate, top with baby spinach, and place the cooked patties on top.
9. Dollop the avocado crema generously over the patties and serve immediately.