Combine first 10 ingredients, stirring with a whisk.
Heat oil in a large nonstick skillet over medium-high heat. Sprinkle tuna with salt. Combine sesame seeds in a shallow dish. Dredge tuna in sesame seeds. Add tuna to pan; cook 3 minutes on each side or until desired degree of doneness. Garnish with green onions, if desired. Serve tuna with sauce.
Wasabi comes in different intensities. You may want to experiment with several brands to find the right amount of heat for your taste buds. Some of the sesame seeds might pop out of the pan as the tuna cooks, so use caution and wear an oven mitt as you turn the fish over.
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