Cut the chard crosswise in ½-inch slices (small spinach, lamb’s quarters and amaranth leaves can be left whole). In a very large (12-inch) skillet, heat the oil (or its stand-in) over medium-high. Add the onion and cook, stirring frequently, until browned but still crunchy, about 4 to 5 minutes. Add the garlic and chile flakes, stir for a few seconds until aromatic, then add the broth or water, ½ teaspoon salt and the greens. Reduce the heat to medium-low, cover the pan (if you don’t have a lid, a cookie sheet works well) and cook until the greens are almost tender, anywhere from 2 minutes for tender spinach and amaranth greens to 7 or 8 minutes for thick collard greens—Swiss chard needs about 5 minutes.
Uncover the pan, raise the temperature to medium-high and cook, stirring continually, until the mixture is nearly dry. Taste and season with additional salt if you think necessary.
Serve with warm tortillas, salsa and crumbled cheese for making soft tacos.