In the bowl of a food processor, combine the butter, egg yolk, lemon juice, mustard, bread crumbs, green onions, parsely, and garlic. Pulse for 1 to 2 minutes to throughly combine into a paste. Pour the butter mixture onto a piece of plastic wrap, roll the wrap as you would a cigar, into a cylindrical shape, and refrigerate for 2 to 3 hours until hardened.
Cut the halibut into four to six equal steak portions.
Brush the soaked wood with oil and place into the oven on a baking sheet for 10 minutes to begin to season the wood. Brush the fish with oil ands season with salt and pepper.
Remove the hardened butter from the refrigerator, unwrap, and cut into 1/4-inch thick slices. Place 1 or 2 slices on top of each piece of fish. Place each halibut steak on top of a hot cedar plank and put the planks into a roasting pan large enough to hold them all side by side. Put the roasting pan into the oven and cook for 8 to 10 minutes, or until the fish is cooked to the desired degree of doneness and the butter has melted into a browned crust. Remove from the oven and serve immediately.