In a large deep skillet or wok, heat the oil over high heat until it shimmers. Add the scallions and cook, stirring frequently, until they just begin to brown on the edges, about 2 minutes. Add the greens in handfuls, tossing to coat in oil. As they begin to wilt, keep adding them by handfuls. Once all the greens are in the pan, keep stirring and tossing constantly, until they darken and begin to brown and crisp on the edges. Taste to see if you like the texture. (I prefer mine with some crunch, but cook them the way you like them.) Season to taste with salt and pepper. If you want a little heat, add a pinch of red pepper flakes or a dash of hot sauce or hot pepper vinegar. Serve hot with lemon wedges.