The 1/3 cup soy sauce and maple syrup should be combined. Pour the mixture over the fillets in a sizable glass pan. Put the fillets in the fridge.
Add any leftover maple syrup and soy sauce to the small pot. Simmer for two to three minutes to thicken.
Put in horseradish. Discard the glazing. Set the grill to high heat. Take the fillets out of the fridge, then season with freshly ground black pepper.
Place the fillets flesh side down on the grill after thoroughly lubricating it with vegetable oil. For 3 minutes, grill the fillets. Turn the fillets gently, then coat them all with glaze. 3 minutes more of cooking is required.
Add more glaze using a brush, then reduce heat to low and cook the fillets until they are almost flaky. Brush with any remaining glaze.