Tip: Save leftover onions in fridge for up to 2 weeks. Use in sandwiches, on pizza or stirred into sautéed vegetables.

1. In heavy-bottom pot or cast iron skillet over medium heat, melt butter; add onions, miso and sugar. 
2. Cook, scraping the bottom occasionally, until onions are browned, about 20 minutes. 
3. Stir in mustard. 
4. Transfer to container and stir vigorously until mixture comes together.
5. Preheat oven to 400F. 
6. Line baking sheet with parchment paper. 
7. Arrange salmon portions onto sheet pan; top each piece with a heaped spoonful of caramelized onions. 
8. Sprinkle panko on top. 
9. Bake until salmon is just cooked through, about 8-10 minutes. Top with sliced chives and serve immediately.