In a large bowl, toss the coleslaw mix with the dressing; divide among 4 plates. Place the sesame seeds on a plate and season with pepper. Gently press the tuna in the sesame seeds to coat each side. In a heavy skillet, heat the oil over medium-high heat. Add the tuna to the hot pan and sear for 3 minutes on each side for medium-rare. Slice the tuna and serve over the slaw.