Put whole tuna fillets into a pot and salt heavily, bring to boil then turn down and simmer for an hour (do it outdoors because it will stink out your house). Skim any scum off that floats to the top.

Put the cooked fillets on a rack or tray and leave in fridge to dry out overnight.

Get some old jars (smaller are better for portions), check the rims aren't chipped and the rubber under the lid isnt peeling or damaged. Give them a rinse out and make sure there isnt any crud stuck to the rims or under the lids.

Put any flavours into the bottom of the jars to taste, piece of chilli, peppercorns, bay leaf etc

Break the tuna into the biggest possible sizes you can manage to fit into the jars at once. Don't fill all the way to the top, leave at least a couple of centimetres so the oil will cover the tuna and there will still be a cm at least of air gap to the lid.

Fill with extra virgin olive oil so the tuna is covered, you may need to fill them all then go back and keep topping them up as the oil works in and the air comes out. You can use mostly olive oil and a bit of sunflower oil to make it cheaper and a milder flavour.

Wipe the rims clean and under the lids then put the lids on tightly.

If you have a pressure cooker you can put them in and add water to about half way up the jars then bring up to pressure and turn down heat to low and cook for 45 minutes. Without pressure cooker put them in a large pot with water at least half  up the sides and bring water to a vigorous simmer. Better with a lid on to stop evaporation. Keep topped up with hot water. Cook for 1.5 hours, turn off heat and let them cool down.

Put the jars on a bench and check the lids. When they are still hot they will bulge upwards and as they cool they will suck the lids down under vacuum. If the lids pop up and down when you press them after they have fully cooled the seal didn't take and it's probably a shit jar/seal. Sometimes you can get lucky and undo the lid, clean it, reseat it and boil again. If that doesn't work just keep it in the fridge.

Let them sit at least a month before eating. Enjoy.