Mix sesame oil, lemon juice, soy sauce, brown sugar, sesame seeds, ground mustard, ginger, and garlic powder in a small saucepan over low heat. Bring to a simmer, stirring until sugar has dissolved. Set aside 1/2 cup of marinade for basting. Pour remaining marinade into a resealable plastic bag. Add salmon steaks, coat with marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 1 to 2 hours. Set an oven rack about 6 inches from the heat source and preheat the oven's broiler. Broil salmon steaks under the preheated broiler for 5 minutes, brushing salmon with reserved marinade. Turn and continue to broil until salmon is opaque and flakes easily, about 5 more minutes. Brush with marinade.