In a saucepan over medium-high heat, combine vinegar and sugar; bring to a boil then reduce heat and simmer until sauce has reduced by half; set aside.
Meanwhile, sprinkle salmon with salt and pepper.
Heat the oil in a large skillet over medium heat; add salmon steaks and cook for 4 to 6 minutes per side.
Place spinach in a large bowl; add the warm balsamic sauce, sprinkle with salt and pepper to taste and toss well.
Evenly arrange wilted spinach on dinner plates and top with salmon steaks; serve.