Preheat the oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil, and grease the foil with cooking spray.
Combine the onions, vinegar and brown sugar in a saucepan over medium heat. Cook, stirring occasionally until the sauce begins to caramelize, about 15 minutes.
Melt butter with garlic in a small skillet over medium heat.
Lay salmon fillets on the prepared baking sheet, and brush with garlic butter.
Pour the onion mixture over the fillets.
Bake for 20 to 25 minutes in the preheated oven, until the fish flakes easily. Cooking time may vary with the thickness of your fillets.
