Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.
In a bowl, toss the tomatillos and red onion with the olive oil, and salt and pepper to taste. Spread on the prepared baking sheet and top with the salmon fillet. Season the salmon with salt and pepper and top with the lemon slices, chopped chives and chopped cilantro. Roast 20 to 30 minutes for rare to medium rare. Transfer the salmon to a serving platter and return the tomatillo and onion mixture to the oven. Continue to roast until tender and caramelized, about 5 minutes more.
Serve the tomatillos and onions alongside the salmon.