In a medium pot of boiling water, cook the lentils, barley and garlic clove (if you like) for 40 minutes, or until tender. Drain well in a colander and discard the garlic clove.
Let cool completely, then toss in a bowl with the tomato, spinach or parsley, feta, onion and a good grinding of pepper. In a small bowl or jar, whisk or shake together the oil, balsamic vinegar, sugar and mustard; drizzle over the salad and toss to coat. Serve immediately or refrigerate until it's time to eat.