OSLO (Reuters) - If cod soaked in caustic soda, half-rotten trout or fried sheep's head excites your gastric juices, then maybe you should consider a gourmet trip to Norway in December. Most specialities are still prepared by methods dating back six or seven hundred years, when all supplies had to be conserved and stored. Top of the menu of December delights is "lutefisk" or lye fish, which any self-respecting Norwegian will have at least once before Christmas. Lutefisk is rehydrated dried cod soaked in a strong alkaline solution for several days until the fish is soft enough for a finger to be pressed through without meeting resistance. Documents dating back to the Middle Ages tell that the solution was made from the ashes of birch tree. After soaking, the cod is rinsed for several days in running cold water before cooking is completed by either steaming or poaching. The result is a translucent golden colour fillet with a stiff-jelly consistency. Lutefisk tastes surprisingly bland and is usually enhanced by sauces, which differ depending on region. Perhaps the most testing of Norwegian Christmas delicacies is "rakefisk", meaning half-fermented trout, a speciality from the mountain areas of eastern Norway. In August and September, freshwater fish were caught, wrapped in birch bark and buried. The fish would remain underground until the first snow, when they were dug up, rubbed with salt and packed tightly belly up in wooden containers with heavy wooden boards placed on top. Today, the fish are stored in a similar fashion but many people prefer to buy ready-prepared rakefisk because of the risk of bacteria leading to food poisoning. There are many different types of rakefisk, depending on the strength of the brine solution and how long the fish is stored. Pieces of filleted rakefisk are eaten with unleavened potato bread called lefse, with slices of leeks, chopped onion and a thick sour cream. For those still with an appetite and prefering a more meaty meal there's "smalahove", sheep's head -- without the brain but with eyes -- which has been salted, dried and smoked before either boiling or frying. Perhaps understandably, no traditional Norwegian meal is complete without lashings of beer and, for those with a strong constitution, the breath-taking, potato-based aquavit chaser.