Combine soy sauce, brown sugar, cumin, sesame oil and red pepper in a 7 x 11 x 2-inch microwave safe baking dish. Add fish and turn to coat both sides. Let marinate 5 to 15 minutes.
Turn steaks over and cover with lid or wax paper. Microwave at 100% power 1-1/2 minutes, rotating dish 1/4 turn. Sprinkle with onions and continue to cook 1-1/2 minutes longer or until fish begins to flake easily when tested with a fork. Let stand, covered for 2 minutes. Serve fish and sauce over rice.