Choose a dish that will fit the cod and sprinkle salt into it 1.5cm deep. Feel along the fish to see if there are any bones, pull them out out if there are. Put the cod on top, skin-side down. Sprinkle another 1.5cm salt over the top, cover and chill for 24 hours. Remove the salt cod, rinse and soak for about 12-24 hours, changing the water until it doesn’t taste too salty. Cut the cod into four fillets.
Heat oven to 180C/160C fan/gas 4. Pour half the stock into a pan with the carrots, and the other half into a pan with the swede. Add enough water to just cover the vegetables in each pan. Cook each one until the vegetables are almost completely tender and the liquid has been absorbed (the swede will cook more quickly than the carrots), this can take up to 15 mins depending on the size of your pans. Put the vegetables in a small gratin dish. Season. Heat the cream until boiling, then add the parsley, mustard, seasoning and lemon juice. Pour over the vegetables and bake in the oven for about 20 mins. The top should be golden.
Meanwhile, put the cod fillets in a sauté pan and cover with water. Bring to the boil and immediately turn the heat down low and poach for about 8 mins, or until the cod is tender (salted cod never gets as soft as fresh cod).
Heat the butter in a pan and add the bacon. Sauté until golden and the fat is crisp, but be careful not to let the butter burn. Divide the vegetables between four plates, set the cod on top and spoon over the bacon and butter.