Soak the untreated cedar plank fully submerged in cold water for at least 1 hour (up to 2 hours) to prevent burning.
Preheat a charcoal or gas grill to medium-high heat (about 400°F/200°C), ensuring the grates are clean.
While the plank soaks, pat the redfish fillets dry with paper towels to remove excess moisture.
Lay the redfish fillets skin-side down and season both sides with salt and freshly ground pepper to taste.
In a bowl, mix the Dijon mustard and brown sugar until fully combined into a paste.
Evenly spread the Dijon mustard and brown sugar mixture over the top (flesh side) of each seasoned redfish fillet.
Place the soaked cedar plank directly on the preheated grill and close the lid, allowing it to heat for 2-3 minutes until it begins to smoke and crackle.
Carefully place the prepared redfish fillets, skin-side down, onto the hot plank.
Close the grill lid and cook for 12-18 minutes, or until the fish flakes easily with a fork and the top caramelizes slightly.
Monitor the grill temperature and check occasionally for any flare-ups around the edge of the plank; adjust grill temperature if necessary.
Once cooked, use tongs and a spatula to transfer the cedar plank with the fillets to a heatproof surface.
Rest the redfish for 2-3 minutes to allow juices to settle.
Serve directly from the plank or carefully lift fillets onto plates.
Suggest pairing with a crisp green salad or grilled seasonal vegetables.
For plating, garnish with fresh cracked pepper and an optional lemon wedge for brightness.