Season both sides of tuna fillet with kosher salt.
In a small dutch oven, add olive oil, thyme, lemon zest, bay leaf, garlic, and peppercorn. Heat oil to 185°F and cook for 20 minutes to allow oil to infuse with aromatics.
Lower heat to a gentle simmer. Gently add tuna to the oil and bring oil temperature back to 150°F.
Turn off heat. After about 3 minutes, remove one piece of tuna and check for doneness. If you prefer tuna medium rare, remove from oil when center is still pink. For more well-done, keep in oil until desired level of doneness is reached (6-9 minutes).
Place tuna in a dish. Once oil is cooled to room temperature, strain and pour over tuna.
Cover leftover tuna and place in the refrigerator. Good for up to two weeks.