Prepare the Marinade: In a medium mixing bowl, combine ¾ cup soy sauce, ½ cup pineapple juice, 1 tablespoon sesame oil, ¼ cup brown sugar, ¼ teaspoon ground ginger, ½ teaspoon crushed red pepper flakes, 4 minced garlic cloves, and 1 tablespoon ketchup. Stir until the brown sugar is fully dissolved and the marinade is well blended.

Marinate the Salmon: Place the 4-6 salmon filets in a large resealable plastic bag or a shallow dish. Pour the prepared marinade over the salmon, ensuring that each filet is well coated. Seal the bag or cover the dish, and refrigerate for at least 30 minutes, allowing the salmon to absorb the flavors. For a deeper flavor, marinate for up to 2 hours.

Prepare the Grill: Preheat your grill to medium-high heat (about 375-400°F). Lightly oil the grill grates to prevent the salmon from sticking.

Grill the Salmon: Remove the salmon from the marinade, letting any excess drip off. Reserve the marinade for later use. Place the salmon filets on the grill, skin-side down. Grill for 4-6 minutes per side, depending on the thickness of the filets, until the salmon is cooked through and has nice grill marks.

Grill the Pineapple Rings: While the salmon is grilling, place the pineapple rings on the grill. Grill for 2-3 minutes per side, or until they are caramelized and have grill marks.

Thicken the Marinade (Optional): While the salmon is grilling, you can thicken the reserved marinade to use as a sauce. In a small saucepan, bring the marinade to a simmer over medium heat. In a separate bowl, mix 1 tablespoon cornstarch with 3 tablespoons water to create a slurry. Gradually whisk the slurry into the simmering marinade, stirring constantly until the sauce thickens, about 2-3 minutes.

Serve: Remove the salmon and pineapple rings from the grill. Serve the grilled salmon topped with the caramelized pineapple rings. Drizzle the thickened marinade sauce over the top for added flavor, if desired. Enjoy your tropical Hawaiian grilled salmon!