Warm the oil in a pan large enough to accommodate the fish fillets in a single layer.
Season the fillets with salt and pepper and place in the pan.
Seal on both sides over a low heat, without colouring the flesh.
Add the zest, chilli and garlic, and fry gently to allow the flavours to develop, about 1-2 minutes.
Add the wine and milk, increase the heat and bring to the boil. Reduce to a very gentle simmer and cover, with the lid gaping slightly.
Poach the fillets until just cooked through, 4-8 minutes, depending on their thickness. When done, the flesh will flake when pressed gently. The sauce will appear slightly curdled.
Remove from the heat and transfer the fish to warm serving plates. Gently pull off the skin. 8. Add the mascarpone to the pan and stir into the sauce.
Taste for seasoning and add the basil at the last minute.
Serve the cod with creamy mash and plenty of sauce spooned over the top.
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