In a mixing bowl, whisk together the soy sauce, honey, rice vinegar, minced garlic, minced ginger, and sesame oil until well combined.
Reserve ¼ cup of the marinade for basting later. Pour the remaining marinade over the salmon fillets in the mixing bowl. Make sure the salmon is fully coated. Cover and marinate in the refrigerator for at least 15 minutes (up to 1 hour for more flavor).
Preheat your grill or oven to 400°F (200°C).
In a small bowl, mix the cornstarch and water until smooth. Add this mixture to the reserved marinade and whisk until well combined.
If grilling, lightly oil the grill grates to prevent sticking. Place the marinated salmon fillets on the grill, skin side down. Grill each side for about 4-5 minutes, or until the salmon is cooked through and flakes easily with a fork. If baking, place the salmon fillets in a baking dish and pour the reserved marinade over them. Bake for about 12-15 minutes.
In the last few minutes of cooking, baste the salmon with the marinade mixture for added flavor.
Once cooked, remove the salmon from the grill or oven and let it rest for a couple of minutes.
Garnish the salmon with chopped green onions and sesame seeds before serving.