To prepare sauce, combine all ingredients in a sauce pan and bring to a boil over medium heat. Simmer for 4 to 5 minutes to thicken.
To prepare fish, combine Dijon mustard with next two ingredients. Coat fish with mixture. Spread sesame seeds on a plate and roll coated fish in seeds. Heat oil in a large skillet over medium-high heat. Add fish and lightly brown each side. Do not cook fish more than a few minutes. When properly cooked, only the outside 1/8 inch is cooked. Most of the center is still raw (call it “rare” if it makes you feel better).
To serve, spoon sauce onto plates. Carefully slice tuna into medallions and arrange over sauce.