Place the salmon fillets in a baking dish, tightly-sealing container, or zipped-top bag. Whisk the maple syrup, soy sauce, sesame oil, and garlic together in a medium bowl. Pour about half of the marinade over the salmon fillets, turning to coat them thoroughly. Reserve the remaining half of the marinade. Cover the dish/seal the container or bag.

Refrigerate the marinating salmon for at least 15 minutes or up to 8–9 hours. If you aren’t baking the salmon until later, tightly cover the unused marinade and refrigerate it until needed in step 5.

Preheat oven to 375°F (191°C). Line a baking sheet with aluminum foil, parchment paper, or a silicone baking mat. Arrange the marinated salmon fillets on the prepared baking sheet and top each with sesame seeds. Discard the used marinade.

Bake the salmon until the center is opaque and flakes apart easily with a fork, 15–20 minutes depending on thickness (about 10 minutes per 1-inch thickness measured from the thickest part of the fillet, or until the thickest part reaches 145°F (63°C) internal heat). If desired (for crispier edges), broil the salmon on high for the last 2 minutes.

Pour the reserved marinade into a small saucepan or skillet. Bring to a boil, then reduce heat to medium-low and simmer for 5 minutes, until the sauce has thickened into a glaze.

Garnish the salmon with chopped scallion (if using) and serve with the glaze.