Start by patting Tuna loin dry with paper towels. Mix the salt, sugar, peppercorns, and juniper berries together in a bowl, then coat all sides of the Tuna with the spice mixture.
Place Tuna in a vacuum-seal bag or large Ziplock bag, add satsuma slices and whiskey, then seal. If you choose to use a Ziplock bag, try and remove as much air as possible. If using a vacuum sealer, make sure to place it on wet setting or use one size up vacuum bag.
Place Tuna on a baking pan, add second baking pan on top, and weight it down with canned vegetables or a cast iron pan. Refrigerate the Tuna for 48 hours, turning the fish over after 24 hours. (The fish can be stored in the fridge for up to 5 days if needed.)
When ready to serve, remove tuna from the bag. Using the back of a knife, scrape the cure mixture off the fish and slice thinly. Serve with a hearty toasted rye bread, capers, pickled onions, etc.