Bring a large pot of water to the boil and season with salt and cider vinegar. Wrap the roe individually in baking paper and carefully simmer for 5 minutes before letting it cool in the cooking liquid.
Divide the cod into 4 large steaks and season each piece on the meat side with salt and pepper and let them draw for a while.
Trim the leeks and divide the white bottom from the darker green top. Slice the white bit finely and chop the darker top more roughly, washing them separately in lots of cold water.
Sauté the white leek in butter with the thyme until it softens but does not color from the heat. Add ¼ cup (50 ml) cider vinegar and all the apple juice and let this reduce until almost evaporated. Add the cream, heat through and mount the sauce with coarse mustard, seasoning to taste with salt, pepper and more vinegar.
Briefly sauté the leek tops in a little butter in a shallow pot, season with salt and pepper and place the pieces of cod, skin side up, on top. Put a lid on the pot and steam carefully for 5-6 minutes, until the skin peels easily from the cod. Meanwhile, chop the apple and toss with the last of the cider vinegar.
Just before serving, heat up a small frying pan, remove the small sinews from the roe and fry the roe, whole or split, in a little oil.
Season generously and serve the fried roe on the soft white leek, the cod with a little of the greens and scatter the apple over the plate.