Soak cedar planks in water for at least 2 hours, or up to 24 hours, prior to grilling. Remove salmon from fridge 30 minutes before grilling.

Make Glaze: Stir brown sugar and Dijon mustard together until smooth. Season salmon with salt and pepper.

Prep Grill: Preheat grill to medium-high heat. Once hot, set the grill for indirect grilling (so the cedar planks don't catch fire). For a two-burner grill, turn one burner off and place cedar planks on the unlit side. For three or four burner grill, turn off the middle burner(s) and place cedar planks in the center.

Add cedar planks: Place pre-soaked cedar planks on the hot grate, over indirect heat, and close the lid. Leave the plank until there is a smoky smell, about 4-5 minutes.

Rotate planks and place salmon, skin side down, on top. Spread a spoonful of mustard mixture on top of each filet and add a thin slice of lemon, if desired. Close grill lid.

Cook salmon on the cedar planks for about 15 to 25 minutes (depending on thickness), or until cooked through (125 degrees F to 140 degrees; medium rare/medium well). Remove and cover with foil about 5 degrees shy of desired temperature, as it will continue to a cook as it rests on the hot cedar plank. Be careful not to overcook it.