For the glaze: In a small saucepan, bring the vinegar, maple syrup, mustard and garlic to a boil over medium heat. Reduce the heat to a simmer and cook until thick, about 12 minutes. Set aside to cool for 5 minutes.
For the salmon: Position a rack in the center of the oven and preheat the oven to 400 degrees F. Line a small baking sheet with parchment paper or foil. Arrange the salmon on the baking sheet. Drizzle both sides of the salmon with the olive oil and season with 1 teaspoon salt and 1/2 teaspoon pepper. Bake until the salmon is cooked through and flakes easily with a fork, about 8 to 10 minutes. Set aside to cool for 10 minutes.
To serve: Place a bed of fresh spinach on your plate, top with a piece of salmon and drizzle with the glaze. Enjoy!