In a small bowl, whisk the first five ingredients; set aside. Spritz tuna with cooking spray. Sprinkle with sesame seeds, salt and pepper. In a large nonstick skillet, brown tuna on all sides in batches until medium-rare or slightly pink in the center; remove from the skillet.
On each of 30 wooden appetizer skewers, thread one tuna cube. Arrange on a serving platter. Garnish with onions. Serve with sauce. Yield: 2-1/2 dozen (1/2 cup sauce).