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.