Preheat oven to 180°C. Place potatoes in a microwave-proof bowl with 2 tablespoons water. Cover and microwave on high for 5 minutes. Drain well.
Place potatoes, courgettes, onions and garlic in a single layer on a large baking tray. Spray with oil and sprinkle with dried oregano. Roast for 15 minutes.
Remove vegetables from oven. Add tomatoes and olives to tray. Place salmon on top of vegetables and season well with black pepper. Return to oven and roast for 12 more minutes or until salmon is cooked to your liking and vegetables are tender.
Remove from oven and garnish with oregano and lemon zest.