Prepare the marinade in a mixing bowl by combining the San-J Gluten Free Tamari, sugar, mirin, sake, ginger and garlic. Whisk to mix well. Place the salmon fillets in a glass pan (or other non-reactive pan) and pour the marinade over the top. Cover tightly with plastic wrap and refrigerate at least 2 hours or up to 4 hours.
Preheat the broiler to high heat. Remove the salmon fillets from the marinade and pat dry. In a saucepan, heat the marinade to boiling and cook for 5 minutes. Season each fillet with salt and pepper to taste. Place the fillets on the broiler pan and broil for 6 to 7 minutes. Baste every 2 minutes with the marinade. Turn and continue broiling for 6 to 7 minutes. Set aside and keep warm.
In a large stockpot or saucepan, combine the chicken broth, San-J Gluten Free Tamari, mirin, rice vinegar, garlic, ginger, sugar and sesame oil and bring to a boil. Simmer for 5 minutes, stirring occasionally. Add the spinach leaves, covering the spinach with the broth and continue simmering for 30 seconds. To serve, ladle the broth and spinach into 4 shallow bowls and place a broiled salmon fillet on top of each serving.