Step 1 Crush the peppercorns by placing them in a thick plastic bag and pounding it with a hammer, mallet or rolling pin.
Step 2 Combine the crushed peppercorns, salt and sugar in a large resealable plastic bag.
Step 3Add the tuna and Armagnac to the bag, shaking to combine. Press out the remaining air, seal and let sit in the refrigerator for 24 hours, shaking the bag once or twice.