Heat 1 tablespoon oil in a 12-inch skillet over medium heat
until hot. Pat salmon dry and sprinkle with 1 tsp. salt and 1/2
tsp. pepper, then cook, skin side up, until undersides form a
golden crust - about 12 - 15 minutes. Turn fish over and cook
until just cook through, about 3 minites more.
Meanwhile, heat remaining 2 tablespoons oil in a large skillet
of medium-high heat until it shimmers, then saute onion until
golden brown and crisp-tender, 6 - 8 minutes. Stir in vinegar,
sugar, and 1/2 tsp. salt and cook until sauce is syrupy, about
2 minutes. Remove from heat and stir in butter.
Spoon onions with sauce onto plates and top with salmon,
skin side down.