Allow the tuna steak to come to room temperature by letting it stand for at least 20 minutes.
Pat the tuna dry. Sprinkle it liberally with kosher salt and fresh ground black pepper on both sides to get a nice even coating (about 3/4 teaspoon kosher salt for 8 ounces).
Heat the oil in a medium skillet over medium high heat.
Add the tuna steak and cook 1 to 2 minutes per side, until lightly browned on the outside but still rare on the inside. The tuna will continue cooking while sitting. For a medium-rare tuna steak, the internal temperature should be 130 degrees Fahrenheit when measured with a food thermometer at the thickest point.
Cool for 2 minutes. Then slice the tuna against the grain into ½-inch slices and serve immediately, with the sauce if desired.
Notes
*Frozen is okay, just thaw it in the refrigerator before serving.