In a small bowl, combine the vinegar, brown sugar, mustard, and salt.
Brush 1/2 of the mixture over the salmon.
Broil or grill until opaque, 4 to 5 minutes, brushing with the vinegar mixture after 2 minutes.
Meanwhile, heat the oil in a large skillet over medium heat. Add the onion and cook until lightly browned, 2 minutes. Cover and cook until tender, 2 minutes. Add the spinach and cook until almost wilted, 1 minute.
Add the remaining vinegar mixture and cook, stirring, until the spinach is wilted and the sauce thickens slightly, 1 minute.
Divide the spinach mixture equally among 4 dinner plates. Top each serving with a piece of salmon.