Cook mustard greens in 2 batches in a 6 to 8 quart pot of boiling salted water, stirring to submerge, until wilted and tender, about 5 minutes. Transfer to a large bowl of cold water to stop cooking. Drain greens in a colander, pressing to squeeze out excess moisture, then coarsely chop.
Cook garlic in butter in a 4 to 6 quart heavy pot over moderately low heat, stirring until softened, about 2 minutes. Add boiled greens, salt and pepper and cook, covered stirring occasionally until heated through, about 5 minutes.