Mix ground mustard and dijon mustard in a small bowl to create a paste.
Coat the Ahi tuna steaks with soy sauce and the mustard paste evenly on all sides.
Combine black and white sesame seeds in a shallow dish.
Press the coated tuna into the seeds until thoroughly covered.
Heat canola oil in a pan over medium-high heat until hot.
Place the tuna in the pan and sear each side for about 1-2 minutes.
Remove the tuna from the pan.
Allow it to rest for a minute before slicing into thin pieces.
Serve with lime juice and fresh ginger on the side.