Pulse the black beans, lime juice, cumin, and a few shakes of hot sauce in a food processor until it has the consistency of refried beans, adding a bit of water to help if necessary. Coat a small nonstick pan with nonstick cooking spray or a bit of butter or olive oil and heat over medium heat. Crack two eggs into a bowl and beat with a bit of salt and pepper. Add the eggs to the pan, then use a spatula to stir and then lift the cooked egg on the bottom to allow raw egg to slide under. When the eggs have all but set, spoon a quarter of the black bean mixture and 2 tablespoons feta down the middle of the omelet. Use the spatula to fold over a third of the egg to cover the mixture in the center, then carefully slide the omelet onto a plate, using the spatula flip it over at the last second to form one fully rolled omelet. Repeat with the remaining ingredients to make four omelets. Garnish with pico de gallo, avocado slices if you like, and bit more crumbled feta.