@Migueldeicaza my recipe is a bit different than what you linked.
I add kosher salt and pepper to the tuna, sear in a heavy stainless steel pan with a little bit of neutral oil. Sear on both big sides, then briefly on the edges holding it with tongs to get them browned
Then I deglaze the pan with a sauce made from soy sauce, sake or Chinese rice wine, mirin, maple syrup, splash of rice vinegar, splash of toasted sesame oil, garlic, ginger, and scallion. Reduce until thickened.