Pat dry tuna steaks using a paper towel, and season both sides with salt, onion powder, cayenne pepper, and black pepper.

Heat a medium skillet over medium-high heat for 3 to 5 minutes until very hot.

Add butter and garlic cloves to the hot pan. Once the butter has melted, add the rosemary sprigs, followed by the tuna steaks. Sear both sides of the tuna for 1 to 1½ minutes for medium-rare; you can cook for more time according to your desired doneness.

Once the tuna is cooked, remove it from the pan and transfer it to a cutting board. Let the steaks rest for a few minutes.

Slice the tuna steaks into ½” slices, garnish with toasted sesame seeds, green onions, and a squeeze of fresh lime juice, and serve.