Preheat oven to 400 degrees.
In a small bowl, whisk together soy sauce, lime juice, rum (if desired), broth, garlic and brown sugar until brown sugar has dissolved; set aside.
Place salmon fillets in a baking dish and pour sauce on top; evenly spread sliced onion over the sauce.
Bake for 20 to 25 minutes or until fish flakes easily when tested with a fork.
Garnish with cilantro and enjoy!