Mix sesame oil, lemon juice, soy sauce, brown sugar, sesame seeds, ground mustard, ginger, and garlic powder in a saucepan over low heat. Bring to a simmer, stirring until sugar has dissolved. Set aside 1/2 cup of marinade for basting.
Pour marinade not for basting in a plastic bag with the salmon. Squeeze air out of the bag, seal, and marinate the salmon steaks for at least 1 hour.
Preheat the oven’s broiler. Place salmon steaks into a broiler pan and broil for 5 minutes. Brush steaks with reserved marinade, turn, and broil until fish is opaque and flakes easily, about 5 more minutes. Brush again with marinade and serve.