1. Preheat oven to 425 degrees F. Spray a rimmed baking sheet with vegetable cooking spray.
2. Place potatoes and cauliflower on baking sheet. Drizzle with oil, tossing to coat, then sprinkle with salt, pepper, and minced garlic. Toss to coat all pieces completely. Roast until vegetables are lightly browned and tender, about 25 to 30 minutes, stirring every 10 minutes to turn the veggies.
3. Remove from oven and let cool for 10 minutes, then transfer to a large bowl. Add celery, scallions, onion, and parsley.
4. In a small bowl, combine mustard with lemon zest and juice. Add to vegetables and toss to coat. Serve warm, at room temperature, or cold.
Source: Hannaford fresh Magazine, July - August 2013