Preheat the oven to 375 degrees F.
Brush the bag liberally inside and out with the olive oil.
Wash the fish well inside and out, then pat dry with paper towels. Season the fish with salt and pepper inside and out. Sprinkle the fish with garlic and rosemary inside and out, then put it in the bag and close tightly. Place the bag on a baking sheet and bake it in the oven for 30 minutes. Remove the sheet from the oven and allow the fish to rest inside the bag for 30 minutes before opening (Be careful when you open that the steam does not burn your hand). Serve with lemon wedges on the side