In a small dry skillet, toast the cumin over low heat, stirring, until fragrant, about 2 minutes. Transfer to a bowl and add the lemon juice and mustard. Slowly stir in the oil until emulsified and season with salt and pepper. Toss the dressing with the barley salad, cover and refrigerate overnight to allow the flavors to blend. Let return to room temperature, then season with salt and pepper and toss before serving.