Lightly season the fish and put it to one side. Heat the oil in a large saucepan or casserole and gently cook the shallots and garlic for 5 to 10 minutes, without browning them.
Add the turnips and potatoes, turn them in the oil and then add the saffron and the liquid. Bring to the boil and simmer until the vegetables are barely tender. Place the cod fillets on top of the vegetables, pa