COMBINE ingredients for salsa. Stir to coat. Refrigerate until serving.
WHISK together ingredients for sauce.
COMBINE sesame seeds, sea salt and black pepper on a plate. Pat tuna steak dry and coat each side in sesame seed seasoning. Heat oil in a skillet over medium/high heat until shimmering hot. Sear tuna 1 minute, flip and sear an additional minute. Sear sides for 15-20 seconds. Set on a serving plate. Serve with cucumber salsa and dipping sauce/dressing. For fully cooked tuna steak, cook each side 4 minutes.