Place the cedar plank on the grill over 350-400 F.
Make the rub: Combine mustard, sugar, a pinch of salt, and pepper together. Mix well.
Coat the salmon fillets with the rub evenly. Add Lemon slices on top of the salmon fillets.
Place salmon on top of the cedar plank. Cover and let it cook for about 15 minutes depending on the degree of doneness. Check occasionally to make sure the plank is not on fire.