Spray a large saucepan with oil. Heat over medium heat. Cook onion, stirring, for 5 minutes or until softened. Place stock powder and boiling water in a large heatproof jug. Stir to dissolve stock. Add mixture to pan. Bring to the boil.
Add chicken and corn. Bring to the boil. Reduce heat to medium-low. Simmer for 10 minutes or until chicken is just cooked through. Remove chicken and corn. Set aside for 5 minutes to cool slightly.
Meanwhile, add pasta to pan. Simmer for 10 minutes or until pasta is tender. Using 2 forks, shred chicken. Cut kernels from corn cobs. Return chicken and corn to pan. Add zucchini. Simmer for 2 minutes or until zucchini is tender.
Remove pan from heat. Stir in spinach until just wilted. Divide soup between bowls. Sprinkle with chives.
Meanwhile, make herb toasts. Preheat grill to high. Place bread on an oven tray and grill for 1-2 minutes, until lightly toasted. Remove from grill. Mix butter and coriander together and spread over untoasted side of bread. Scatter over cheese. Return to grill for 3-5 minutes, until bubbling and golden.