Wash the potatoes and slice into wedges. Place in a large baking dish with the olive oil and cook for 30 mins.
Add the red onion, courgettes and leeks and place the fish on top. Cut 1 lemon into 4 pieces and place around the dish.
Juice the other lemon and whisk with 2tbsp of olive oil. Pour over the dish and sprinkle over the rosemary sprigs.
Cook for 20 mins.