For the Balsamic Onion Glaze
Heat oil in a large sauté pan over medium heat and add onions.
Sweat onions, stirring often, until all liquid had evaporated, about 20 minutes, Add half of balsamic vinegar and simmer until absorbed.
Add remaining balsamic and reduce until a glaze. Season to taste and set aside.
For the Salmon
Preheat a grill to medium-high.
Reduce balsamic with rosemary in a small saucepan to a glaze consistency, about 8 minutes and set aside.
Brush salmon fillets lightly with olive oil and season. Grill skin-side up, for 4 minutes, then rotate 90 degrees and cook 4 more minutes.
Turn salmon over and cook for 8 more minutes for medium doneness. Brush salmon with balsamic during last 5 minutes of cooking.
Serve salmon with Balsamic Onions on the side.