Heat the oil in a medium-size nonstick skillet over medium heat. Add the onion and pepper flakes and sauté until the onion is almost translucent, about 2 minutes. Add the greens and stir to coat with the oil. Sprinkle with salt and pepper and sauté until almost wilted, about 30 seconds for baby greens, 2 minutes for chard or broccoli rabe. (If using kale, sprinkle with water, cover and cook until wilted, about 4 minutes). Make an opening between the greens and break the egg directly into the skillet. Sprinkle with salt and pepper. When the bottom of the white turns opaque, stir the egg to break up and mix egg and greens together. Divide the mixture between the warm tortillas. Sprinkle with cheese and enjoy.