Season the fish with salt and freshly ground black pepper.
Optional - I marinated the blue cod fillets with olive oil, a squeeze of lemon juice and a chopped garlic clove and set aside before cooking the veg.
Heat the oil and butter in a large sauté pan - deep enough to hold all the ingredients.
Add the onion and cook for 3-4 minutes until softened. Add the garlic and continue cooking for 1-2 minutes.
Add the potatoes and sauté, stirring occasionally until they start to colour and soften. Add extra oil and butter as required.
Add the leek and red pepper and cook, stirring gently for a further 5 minutes. Add the herbs and chilli flakes.
Stir in the wine and water, bring to a simmer, season and continue cooking until the potatoes and vegetables are almost cooked.
Add the fish, cover and reduce the heat to low. The cooking time will depend on the type and thickness of the fish. The small blue cod fillets cooked in approximately 10 minutes.
The fish can also be browned in a separate pan before adding to the main dish.
Once cooked, check season, add a squeeze of lemon juice and serve with the remaining lemon wedges on the side.