3 TBSP olive oil, divided
4 (6-ounce) salmon fillets (1 inch thick) with skin
2 medium red onions (about 1 pound total), each cut into 8 wedges
2/3 cup balsamic vinegar
1 tsp sugar
1 TBSP unsalted butter
Heat 1 tablespoon oil in a 12-inch nonstick skillet over medium heat until hot. Pat salmon dry and sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper, then cook, skin side up, until undersides form a golden crust, 12 to 15 minutes. Turn fish over and cook until just cooked through, about 3 minutes more.
Meanwhile, heat remaining 2 tablespoons oil in a large skillet over medium-high heat until it shimmers, then sauté onions until golden brown and crisp-tender, 6 to 8 minutes. Stir in vinegar, sugar, and 1/2 teaspoon 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.
Notes: Somewhat reminiscent of Tuna Trapani Style. I tried this because I scored the Sockeye Salmon on sale again. Still very impressed with the texture of the fish. This had a lovely, tangy taste. Will definitely try again.