Preheat oven to 425°. Place salmon on a baking sheet lined with foil, skin side down.
Mix together mayonnaise, parmesan, lemon zest, parsley a pinch of salt and pepper to taste.
Coat lightly salmon with about two teaspoons of lemon juice. Sprinkle with additional salt and pepper. Coat the top and sides of the salmon completely with the mayonnaise mixture.
Bake salmon in the oven for about 15 minutes or until salmon reaches desired temperature, see note.
While salmon is cooking heat oil in a medium sized sauté pan. Add garlic and black pepper, sauté for about 30 seconds, or until you can smell the garlic develop. Add spinach in batches with a pinch of salt each, toss to wilt. Cover with a lid to encourage wilting if needed.
Optional Step to finish salmon: During the last two to three minutes of baking switch to the broiler to finish the salmon in order to make the topping extra brown and bubbly. Watch closely to prevent burning.
Once salmon is done, place spinach on a platter. Slide a spatula between the skin of the salmon and the flesh and place on top of spinach. You may break it into pieces or leave intact. Serve with lemon slices if desired.