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 at the last minute.
Stir through the stock, season well, cover and simmer for 10 minutes. Halfway through, top with the courgette slices.
Place the fish fillets on top of the courgettes. Sprinkle over the dried herbs, 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.
Chef's tip: try swapping the white fish fillets for vegetarian meat-free fillets. Fry the fillets separately in a pan and serve with the potatoes and courgettes.