Preheat oven to 450°F (230°C). In a bowl, toss asparagus, onion, celery and garlic with olive oil. Season to taste with salt and pepper. Place on a baking sheet and roast, uncovered, until just beginning to brown, about 10 min. Shake pan once during cooking to turn vegetables.
Transfer vegetables to a medium saucepan over medium-high heat. Stir in flour. Stir in broth. Bring to a boil, reduce heat and simmer 10 min. Carefully pour into a blender and purée until smooth. Return to saucepan and heat through. Season to taste with salt and pepper. Spoon 1 tbsp (15 mL) tzatziki over each serving and sprinkle with cracked black pepper.
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