Preheat oven or grill to 350 degrees. In a small bowl, whisk together sesame oil, tamari and ginger.
On 4 large pieces of foil place salmon fillets in the centre. Lightly turn up the ends of the foil around the salmon and drizzle the oil and tamari mixture evenly between the fillets.
Put spinach and onion on each piece of salmon equally. Wrap salmon fillets with the foil. Place the foil wrapped salmon on a baking sheet and cook for 10 minutes or until the flesh of the fish flakes easily with a fork.