In a medium bowl, combine oil, rosemary, garlic, salt, and pepper. Add tuna pieces and toss to combine. Cover and marinate overnight or up to 24 hours.
Pour 1 cup of water into the pressure cooker and place steamer rack inside.
Transfer tuna pieces and marinade to a 1 quart ramekin. Add enough olive oil to cover the tuna.
Cover the ramekin tightly with aluminum foil.
Cook on LOW pressure for 5 minutes.
Release the pressure manually, remove lid, and let it cool for 10 minutes inside the pot.
Lift the ramekin out of the pressure cooker and place it on a wire rack. Remove the foil.
Use a slotted spoon to transfer the tuna to a serving platter if using right away. Save the oil and use it to drizzle on bread, pasta, roasted vegetables, or for cooking.
If not using right away, transfer the tuna and olive oil to an airtight container and keep it in the fridge for a week.