Whisk together the marinade by combining 1/3 cup maple syrup, 1/4 cup soy sauce, 1 Tbsp sesame oil, 2 pressed or finely chopped garlic cloves, and 1/2 tsp freshly ground black pepper until smooth.
Place the salmon portions in a large zip-top bag or shallow dish and pour in the marinade; toss to coat, seal the bag removing excess air, and refrigerate for 45 minutes, turning or flipping the bag halfway through.
Preheat the grill to medium-high heat.
Remove the salmon from the marinade to a platter, reserving the marinade for the glaze, and oil the grill grates using a paper towel held with tongs.
Grill the salmon skin-side-down over medium-high heat for about 4 minutes per side for 1"-thick fillets (adjust time per thickness), carefully flip once, and continue grilling until the fish is opaque and flakes easily with a fork.
While the fish cooks or after removing it, bring the reserved marinade to a boil in a small saucepan over high heat and whisk constantly for 3–4 minutes until it thickens slightly into a glaze.
Brush the grilled salmon with the reduced soy-maple glaze, garnish with finely chopped green onion, and serve.