Press the tuna into the sesame seeds, coating completely on both sides. It is up to you if you coat the sides as well but it will make cooking it a bit more difficult so I recommend just coating the top and bottom.
Heat a cast-iron skillet over medium-high heat until it is very hot. Once hot, add the oil to the pan and quickly swirl. Add the tuna and cook for about 45-60 seconds until the sesame seeds are golden brown. Carefully flip over ad repeat on the other side for about one minute. This quickly seared version will yield a tuna that is seared and crisp on the outside and rare on the inside. This recipe doesn't work well for well-done fish since the sesame seeds will burn. Also, be careful while cooking because the pan will spatter. I recommend having the fan on high. Let the tuna rest and then slice thin to serve.
Dipping sauce: I love to serve this with a fresh and zingy dipping sauce. Combine 1/4 cup low sodium soy sauce with 1 tbsp rice vinegar, 1 tbsp sesame oil, 1-2 teaspoons honey, 1 tsp minced ginger, 1 tsp minced garlic, and 1 tbsp minced green onion. Refrigerate until ready to serve.