Combine first 3 ingredients in a zip-top plastic bag; add salmon. Seal bag; marinate in refrigerator 30 minutes.
Preheat oven to 500°.
Remove fillets from bag, and discard marinade. Place the fillets, skin sides down, in a shallow roasting pan coated with cooking spray; cook at 500° for 13 minutes or until fish flakes easily when tested with a fork.
Heat the oil in a Dutch oven over medium-high heat. Add garlic, salt, and one package of spinach; sauté 1 minute or until spinach wilts. Add remaining spinach, and cook 2 minutes or until the spinach wilts, stirring frequently. Remove from heat; toss with rice vinegar. Arrange 2/3 cup spinach mixture on each of 4 plates; add 1 salmon fillet. Garnish with onions, if desired.