This sauce looks like a six-hour job, with bone-roasting, vegetable chopping and long, slow reduction — the antithesis of the minimalist ideal. But, because it’s based on caramelized sugar, it’s actually relatively simple.
Preheat oven to 450 degrees. Place sugar in heavy-bottomed saucepan, preferably nonstick and with rounded sides, and turn heat to medium. Cook without stirring (just shake the pan occasionally to redistribute sugar) until sugar liquefies and begins to turn brown, about 10 minutes. Turn off heat, and carefully add wine. Turn heat to high, and cook, stirring, until caramel dissolves again. Add rosemary sprig, and cook over high heat, stirring occasionally, until mixture is syrupy and reduced to just over 1/2 cup, 10 to 15 minutes.
As liquid reduces, heat a nonstick skillet over high heat until it begins to smoke. Season salmon on both sides with salt and pepper, then place in pan and immediately transfer to oven. Cook 3 minutes, then turn salmon and cook another 3 minutes. Remove salmon when medium-rare or thereabouts (or cook another minute or two if you like it more done), and keep warm.
When sauce is reduced, stir in balsamic vinegar and butter and turn heat to medium-low. Cook until butter melts. Season with salt and pepper, and remove rosemary sprig. Taste and adjust seasoning, then serve over salmon, garnished with chopped rosemary.