Heat air fryer to 385°F. Toss onion with ½ tablespoon oil and a pinch each of salt and pepper and air-fry 5 minutes.
Toss with Swiss chard, garlic, 1 tablespoon oil, and ¼ teaspoon each salt and pepper and air-fry until chard and onion are just tender, about 5 minutes more. Transfer to plates.
Season salmon with ½ teaspoon each salt and pepper and air-fry at 400°F until skin is crispy and salmon is opaque throughout, 8 to 10 minutes. Serve with chard and drizzle with chili oil if desired.
