Heat the oil in a large frying pan and cook the onions and potatoes for 10-15 minutes, until starting to brown. Add the garlic for the last minute.
Stir through the stock, season well and cover, then simmer for 5 minutes. Top with the courgette slices and simmer for a further 5 minutes.
Place the fish fillets on top of the courgettes. Sprinkle over the dried herbs and lemon zest and season to taste. Cover and cook for 5-6 minutes, until the fish is opaque and flakes easily. Serve immediately with the potatoes and squeeze over the lemon juice.