Soak cedar planks in water for 2 hours before cooking.
Preheat grill to 450°F.
Season fish inside and out with salt and pepper. Stuff lemon slices, garlic slices, and butter cubes inside each fish. You may have to cut the lemon slices in half depending on the size of your fish.
Place prepared fish on cedar planks and grill for 25 minutes or until the internal temperature reaches 145°F. Time can vary widely depending on the size of your fish.
The fish will be very flaky and tender so don't expect to serve a uniform filet. Remove the lemon slices and garlic. Find the backbone on the head end of the fish and carefully pull it out so most of the bones come with it. Then you can remove the flesh from the skin. There will probably still be some bones remaining in the flesh so be careful when you eat it.
Sauce
While fish is cooking, add butter to small pan and heat over low heat until it becomes bubbly and light brown. It should also produce a nutty smell.
Once butter is browned, carefully pour in white wine a little at a time. If you add it too quickly, the butter will splatter. Add in salt and pepper. Heat over medium-low heat until sauce has reduced by half. Remove from heat and add in lemon juice and parsley. Drizzle sauce over fish after skin and bones have been removed.