Soak cedar plank boards for at least 2 hours in water. (I’ve read that you can soak and freeze boards to use later, but have not tried this method.)
Preheat grill to 450F.
Lay salmon on one or more of the board (depending on how many cuts you have.) Mix lemon juice, honey, and mustard. Brush salmon with sauce, and if desired, top with a lemon slice.
Place board(s) on the grill until salmon reaches at least 140F, around 8-15 minutes*
Video
Notes
If you like your salmon more well done:
Consider cutting one large fillet into smaller pieces for faster more even cooking.
If you need to cook longer than 10 minutes, the edges of your board may start to "ember." Remove board, spray with water to put out the "ember." Place the board on heavy-duty aluminum foil and return to the grill to finish cooking.