Pesto made with arugula leaves gives this salad a pleasant peppery bite.
In a covered large saucepan cook potatoes in enough boiling salted water to cover for 20 to 25 minutes or just until tender. Drain and cool slightly. Peel and cube potatoes. Place potatoes in a large bowl.
For pesto, in a food processor or blender combine arugula, the 1/3 cup cheese, the 1/3 cup pine nuts, the water, lemon juice, garlic, salt, and pepper. Cover and process or blend until nearly smooth. With processor or blender running, slowly add oil through opening in lid in a thin, steady stream, stopping to scrape down sides as necessary.
Pour pesto over potatoes; toss gently to coat. Gently stir in tomatoes. If desired, top with additional pine nuts and/or shaved cheese. Serve slightly warm or at room temperature.
To toast pine nuts, preheat oven to 350°F. Spread pine nuts in a shallow baking pan. Bake for 5 to 10 minutes or until light brown, watching carefully and stirring once or twice.