Step 1Put the greens in a large nonreactive bowl. Sprinkle with salt and mix and squeeze with your hands in the bowl until the greens are softened and the bowl is filled with green juice. Press the greens into the juice and let stand for at least 2 hours at room temperature or cover and refrigerate for up to 1 day.
Step 2Heat the oil in a wok or large, deep skillet over high heat. When the oil shimmers, add the shallot and garlic and cook, stirring, until the shallot is just translucent, about 30 seconds. Add three-quarters of the pickled greens and stir for 15 seconds, then add the fava beans and sugar. Stir for 30 seconds, then add the green juices from the bowl, reserving the remaining greens for serving.
Step 3Add ½ cup water to the wok and cook, stirring occasionally, until the favas are soft enough to mash with a wooden spoon, about 5 minutes. Reduce the heat to low and start mashing the favas with the spoon. As you coarsely smash the favas, they’ll soak up the water. You want the texture to remain like loose, chunky mashed potatoes, so add more water, 1 tablespoon at a time, if the mixture starts to stiffen.
Step 4Transfer the favas to a serving dish and top with the reserved pickled greens. Serve hot.