Step 1 Preheat oven to 180°C. Line a large baking tray with baking paper. Spray bread rounds with olive oil and sprinkle all over with sumac. Roll up rounds and slice thinly. Unroll ribbons and place on prepared baking tray; bake for 3 minutes, or until ribbons are lightly browned and crisp.
Step 2 Meanwhile, separate green silverbeet leaves from pale stalks; set aside. Chop leaves roughly and slice stalks into thin strips.
Step 3 Place a large saucepan over medium–high heat. Add olive oil and heat. Add sliced silverbeet stalks, leek, crushed garlic, dried oregano and lemon zest to pan; cook mixture for 3 minutes, or until soft and fragrant.
Step 4 Add chicken stock to pan with 2 cups boiling water. Bring to the boil, then simmer, uncovered, for 10 minutes. Add chicken to pan; simmer for another 5 minutes, or until chicken is just cooked. Add reserved silverbeet leaves with zucchini; simmer for 2 minutes, or until zucchini is tender.
Step 5 Season soup with cracked black pepper and stir in lemon juice. Serve soup immediately with sumac toast ribbons.