Meanwhile, toss shrimp with 1 teaspoon oil. Coat a grill pan or skillet with cooking spray. Over medium-high heat, heat for a minute or two. Add shrimp and cook until opaque, about 2 ½ minutes per side. Add shrimp to bowl with potatoes. Add tomatoes, celery, green onions and tarragon. In a small bowl, whisk together the remaining 5 teaspoons oil, vinegar, mustard, salt and pepper. Pour dressing over potato mixture and toss well. Serve warm or at room temperature.