To prepare the soy mustard sauce, mix the Coleman’s mustard and hot water in a bowl to form a paste. Let sit for a few minutes to allow the flavor and heat to develop.
Add the vinegar and soy sauce, mix together, and pass through a fine-mesh sieve into a bowl. Cover and refrigerate for at least 1 hour to allow the flavors to develop.
Dredge the ahi in the spice mixture on top and bottom. Heat the olive oil in a nonstick skillet over high heat and sear the ahi for 15 to 30 seconds on top and bottom for rare, 1 minute on each side for medium-rare, or to the desired doneness. Remove the ahi and hold until plating.
Serve the ahi on warmed plates drizzled with the soy mustard sauce and brown rice.