Mix the salt and sugar together. Cut tuna into 4-5 ounce portions and sprinkle all over with mixture. Refrigerate for 6-12 hours.
Rinse and dry the tuna. Place in a shallow pot in one layer. Add lemon zest, peppercorn, rosemary and olive oil. Fish must be completely covered with oil.
Heat tuna on medium heat until just simmering, then reduce heat to medium low and cook 30 minutes or until fish is fully cooked.
Remove tuna from pot to cool. Strain oil and let cool.
When cooled, place tuna and oil in an airtight container or jar. Store in the refrigerator for up to 2 weeks.