In a large pot, bring 2 quarts water to a boil over high heat. Add barley and 1/2 teaspoon salt; reduce to a simmer and cook, uncovered, until barley is tender but still chewy, about 35 minutes. Drain; rinse barley under cold running water until cool. Drain well, and transfer to a large bowl.
Whisk together lemon juice, mustard, honey, and oil in a small bowl; season with salt and pepper. Set aside. Add carrots, cucumber, red onion, dill, and dressing to large bowl with barley. Toss to combine.
At serving time, add watercress and sunflower seeds; toss to combine. Place salad on platter to serve family-style, or mound on individual plates. Serve at room temperature.
For a change of pace, try substituting 3 1/2 cups of another cooked whole grain, such as brown rice or quinoa, for the barley.