Heat a nonstick frying pan on medium-high heat. Swirl in the olive oil. Add the scallops and cook for 1 minutes each side to brown. Remove the scallops from the pan to a plate and tent with tin foil to keep warm.
Turn heat to medium and to the pan, add the butter, mirin, seasoned rice vinegar, soy sauce, miso, mustard and water. Stir well and when the sauce begins to bubble, add the browned scallops back into the pan to finish cooking for 1-2 minutes.
Garnish with toasted sesame seeds and scallion greens.