Preheat the wood or charcoal grill to medium-high heat and put the cedar plank on the grill for about 15 minutes.
Meanwhile, clean the fillets under cold water, pat dry and sprinkle with salt and pepper.
In a small bowl, whisk together the mustard and brown sugar until blended and smooth.
Spread the mustard mixture over the surface of the salmon with a rubber spatula, then turn the fillets over and spread with the rest of the mixture.
Grill, covered, for about 3 minutes. Turn carefully to the other side, and grill about 3 minutes more, or until the salmon is just cooked through and the glaze is beginning to caramelize and turn slightly crusty.
Put the fillet on each plate, and pour the sauce over the redfish.
Serving Suggestions: Serve with rice pilaf and salsa.