Salmon with Agrodolce Sauce

Fancy name, but basically it's just onions sautéed in a sweet balsamic vinegar. Pairing this sauce with the salmon is, of course, delicious! But I've also had leftovers of the onions and sauce with chicken.

olive oil
2 (6 oz) salmon fillets
sea salt
fresh cracked black pepper
1 large red onion sliced in wedges
1/3 C balsamic vinegar
1 Tbls sugar
1 Tbls butter

Pat salmon dry; season with salt and pepper. Cook over medium heat, skin side up, until undersides from a golden crust (10 minutes). Turn; cook until cooked through (another 10 minutes - depending on thickness).
Meanwhile...sauté onions until golden brown. Stir in vinegar, sugar, and a dash of salt. Cook until sauce thickens and becomes syrupy. Remove from heat; stir in butter.
Spoon onions with sauce onto plates. Top with salmon fillets.

I'm still enjoying the sweet yellow onions (not the red, which the recipe calls for). I also used brown sugar.