1. Peel onion and cut into 8 wedges.
2. Pour 1 teaspoon oil into a 2- to 3-quart pan over medium-high heat. When pan is hot, add onion and cook, turning once, to lightly brown, about 5 minutes.
3. Add orange juice and vinegar. Bring to a boil over high heat, then reduce heat, cover, and simmer until onion is very tender when pierced, about 45 minutes.
4. Shortly before onion is done, pour remaining oil into a nonstick 10- to 12-inch frying pan over medium-high heat. When pan is hot, add salmon. Cook, turning once, until fish is opaque but still moist-looking in thickest part (cut to test), 7 to 9 minutes total.
5. Transfer the salmon to plates and serve with onion mixture. Season with salt and pepper to taste.