Combine chopped arugula, bell peppers, celery, onion, and scallions in a large bowl.

Whisk lemon juice, apple juice, oil, mustard, ½ teaspoon salt, and pepper in a small bowl until emulsified. Pour over vegetables and toss thoroughly. Set aside.

Bring about 6 cups water to a boil in a large saucepan and add buckwheat and remaining 1 teaspoon salt. Stir well, adjust heat to maintain a simmer, and cook for about 10 minutes, or until just tender. Drain in a colander or large strainer, and immediately add to vegetables. Toss thoroughly, spread in a serving dish, and garnish with pumpkin seeds. Serve at once.