Cut plank to fit in slow cooker. Using tongs, char the plank over an open flame on both sides for 2-3 minutes. Soak plank for 1 hour.
Place plank inside slow cooker. Season salmon with salt and pepper on both sides and place in slow cooker. Scatter lemon slices on top of salmon. Cook on low for 2 hours.
Throw lemon away
Combine mustard, maple syrup and butter in a sauce pan and melt. Pour over the salmon. Garish with parsley.