Combine the maple syrup, ginger, Dijon mustard, salt, and pepper flakes in a shallow container. Reserve 1/4 cup for basting.

Place fillets in the dish and marinate for one hour.

Preheat a cast iron skillet over medium heat until very hot.

Remove fish from marinade and pat dry.

Place in the hot pan, skin-side down. Let it cook for about five minutes. You will see it gradually becoming opaque. Using a flexible spatula, flip your fillets skin side up. Cook an additional five minutes or until opaque throughout.

Baste each slice with one teaspoon of reserved marinade before serving.

Garnish with chopped scallions.