- Preheat oven to 230°C (450°F). In a bowl, toss asparagus, onion, celery, and garlic with olive oil. Season to taste with salt and pepper.
- Place vegetables on a parchment-lined baking sheet and roast, uncovered, until just beginning to brown, about 10 minutes. Shake pan once during cooking to turn vegetables.
- Transfer vegetables to a medium saucepan over medium-high heat. Stir in flour; then stir in broth. Bring to a boil, reduce heat, and simmer for 10 minutes. Carefully pour into a blender and purée until smooth. Return to saucepan and heat through.
- When ready to serve, spoon 15 mL (1 tbsp.) of tzatziki over each serving and sprinkle with freshly ground pepper.
Fold asparagus stalks over-they'll break off where they start to become tender. Then cut into pieces as indicated.