If desired, prepare mango salsa and set it aside in the refrigerator.
Mix together sesame seeds, salt, and pepper and place on a plate.
Pat tuna dry and drizzle with avocado oil.
Press edges of tuna into sesame seed mixture. Use your hands to pack the seeds evenly and then set the tuna aside on a clean tray.
Heat a 10-inch cast-iron skillet over medium-high heat until it is very hot. Add 2 tablespoons of avocado oil to the skillet and coat the pan Once the oil is hot carefully lay the tuna in the skillet, pressing it down with a spatula.
Sear for approximately 60 seconds or until the side is golden brown.
Carefully flip and seared the second side for 60-90 seconds or until golden.
Sear the edges of the tuna by holding it upright with tongs.
Place the seared tuna on a cutting board and allow it to rest for 2-3 minutes. Using a sharp knife slice the tuna against the grain into approximately quarter-inch thick slices. Serve on white rice and topped with mango salsa.