Stem and wash the greens. If you're using chard, make sure to trim the thicker midribs as well. Steam the greens in a tightly covered pot until wilted. The spinach will be done as soon as it wilts, but the chard will need to steam longer, until it turns dark green. When done, drain the greens and chop them finely.
Heat oil in a large skillet. Add the garlic and sauté over low heat until it just begins to turn golden. Add the scallions and sauté until they soften a bit. Stir in the greens, beans, and seasonings. Cook, covered, over very low heat for 10 minutes.