In a dry skillet on medium heat, toast the white sesame seeds until they are just beginning to turn light golden brown and fragrant. Remove from heat and spread seeds on a plate. Add the black sesame seeds to the plate if using, and stir seeds around with fingertips to combine.
Brush the tuna pieces on all sides with olive oil. Season on all sides with salt and pepper, and dip tuna pieces in the seeds, pressing to coat on all sides. Set aside on a wax foil lined cookie sheet.
When ready to cook, heat the 2 tablespoons olive oil on medium high in the skillet. When oil is hot, add the butter, when the butter foams, add the fish to the skillet. Let cook without moving the fish for about 1 1/2 to 2 minutes, until the sesame crust begins to crisp and brown. When the crust is golden brown, turn the fish and cook until the other side is golden brown and crisp, about another 2 minutes, adding additional butter if needed to keep the fish from sticking and speed up the browning. Once the second side is browned, turn the fish pieces on edge with tongs, to brown the remaining sides.
Remove from heat and set aside. Fish will be medium rare at this point. If you prefer fish more well done, place the ban with the fish in a 375 degree F oven for another 3 to 5 minutes.
You can substitute tuna with either hamachi or bluefish for a similar effect.
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