In a bowl combine soy sauce, mirin, ginger, garlic and sesame oil. Mix well. Place ahi steaks in the marinade and turn to coat. Cover and marinate for 1 hour.
Mix together sour cream, wasabi and vinegar. Cover and refrigerate until ready to use.
Heat olive oil in a large heavy bottomed skillet over medium-high heat. Cook tuna 2 - 3 minutes per side.
Slice thinly and spoon wasabi cream sauce over the top. Serve.