Trim root ends of leeks, ensuring that the layers are still held together. Cut in half lengthwise and rise well. Cut in half lengthwise again. Melt butter in a large skillet over medium heat. Add leeks and cook, turning once, until tender and lightly browned.
Stir together dates and celery. Whisk together oil, mustard, lemon juice and lemon zest; season with salt and pepper.
Place 4 leek quarters on each plate; top with celery and dates. Sprinkle with cheese and drizzle with dressing.
Nutritional analysis per serving: Calories: 370, Fat: 18g, Saturated Fat: 8g, Trans Fat: 0g, MUFA: 7.94, PUFA: 1.44, Cholesterol: 30mg, Sodium: 460mg, Potassium: 164mg, Carbohydrates: 48g, Fiber: 5g, Sugar: 35g, Protein: 8g, Vitamin A: 30%, Vitamin C: 25%, Calcium: 25%, Iron: 10%