Preparation
Cook mustard greens in a 6- to 8-quart pot of boiling water, stirring to submerge, until wilted and tender, about 5 minutes. Transfer with tongs 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 olive oil in a 4- to 5-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, then toss with onions, tomatoes, feta and almonds, add balsamic vinegar and serve.