Roasted Salmon with Sauteed Greens

Ingredients

• 1/4 cup balsamic vinegar
• 2 tbsp brown sugar
• 1 tsp mustard powder
• 1/2 tsp salt
• 1-1/4 lbs salmon (cut into 4 slices)
• 1 tbsp olive oil
• 1 large small red onion
• 8 oz baby spinach