1 small sweet potato, peeled and diced (about 1 1/3 cups)
6 scallions (white and light green parts), thinly sliced
4 small stalks celery, diced
1 medium cucumber, peeled, halved lengthwise, seeded and diced
1 medium apple, cored and diced
Mustard-Dill Dressing (recipe follows)
1/4 tsp. freshly ground pepper
In large saucepan, combine wheat berries and cold water to cover by 2 inches. Cover and let stand at least 6 hours or overnight.
Drain wheat berries. In same saucepan, bring 4 cups fresh water to a boil. Add wheat berries and 1/4 teaspoon salt and return to a boil. Reduce heat to low and simmer until tender, about 30 minutes. Drain, rinse under cold running water and drain well. Set aside.
Meanwhile, in medium saucepan, bring 2 cups water to a boil. Add millet and 1/4 teaspoon salt and return to a boil. Reduce heat to low, cover and simmer until tender, 25 to 30 minutes. Remove from heat and let stand, covered, 10 minutes. Place in large, shallow container, fluff with fork and spread out. Let stand until cool.
While grains are cooking, if using walnuts, preheat oven to 350°F. Spread nuts on baking sheet and bake until toasted, about 10 minutes. Set aside.
In small saucepan, bring 3 cups water to a boil. Add sweet potato and 1/4 teaspoon salt and cook until potato is just tender, about 6 minutes. Drain, rinse under cold running water and drain well.
In large bowl, combine wheat berries, millet, sweet potato, toasted walnuts, scallions, celery, cucumber and apple. Add Mustard-Dill Dressing and toss gently but thoroughly to blend. Season with pepper and remaining 1/2 teaspoon salt. Cover and refrigerate at least 1 hour. Bring to room temperature before serving.