Cook the rice according to the package directions. Meanwhile, pour about 1/2 inch of water into a small saucepan and bring to a simmer over medium-high heat. Fit a steamer basket in pan. Place the green beans in the steamer, cover, and steam until tender, about 4 minutes. Coat the tuna evenly on both sides with the sesame seeds. Heat the oil in a small skillet over medium-high heat. Add the tuna and cook for 1 1/2 to 2 minutes per side for medium rare. Serve with the rice and green beans.