Soak the cedar plank in warm water for 15 minutes. Preheat oven to 425°F. After the cedar plank has soaked, remove it from the water and pat the exterior dry. To make things cleaner I usually roast everything on a sheet pan. Place the plank on the sheet pan and coat the top surface with an even layer of butter.
Pat your fish dry and place it on the cedar plank on a sheet pan. Rub olive oil on the outsides and inside of the fish and season with salt and pepper.
Stuff fish with 2-3 lemon slices depending on how big the fish is. Add the thyme sprigs to the inside of the fish and spread the garlic evenly inside and outside.
Roast the branzino in the oven for 7 minutes. Flip fish over and cook for another 7 minutes. Switch the oven to the broiler setting and cook the fish until the skin starts to blister, about 3-5 minutes.
Remove fish from oven. Drizzle with a little olive oil and sprinkle with some flaky sea salt for garnish. Serve with fresh lemon wedges.