Preheat the oven to 200 degrees F.
In a baking dish just large enough to hold the tuna in 1 layer, place the tuna and sprinkle generously with the salt on both sides. Add the garlic, oregano and marjoram, then add enough olive oil to completely cover the tuna. Cover with aluminum foil and place in the oven. Cook until the tuna is thoroughly cooked and flakes easily, 3 to 4 hours.
Carefully remove the baking dish from the oven and set on a rack to cool completely. Transfer the tuna, with enough oil to cover, to jars, and refrigerate until ready to use, up to 2 months in advance.
Alternatively, tuna may be transferred to sterile mason jars and filled with olive oil within 1/2-inch of the top and processed in a hot water bath for 10 minutes, cooled, and stored at room temperature for up to 6 months. Jars may also be refrigerated for up to 6 months.
This will depend on the thickness of your tuna; you will know that it is done when it flakes easily and the fillets feel firm from the edges all the way to the center.