Spread one bunch of the herbs in a single layer in the bottom of a baking pan that can hold the fish comfortably. Spread the onion slices over the herbs.
Place the fish, skin-side down, on top. Season with salt and pepper. Spread the second bunch of herbs on the fish and top with the lemon slices, arranged in a row. Drizzle the olive oil over all.
Cover and refrigerate for up to 4 hours if not planning to cook immediately. Let it come to room temperature, about 30 minutes, before roasting.
Preheat oven to 500° F.
Roast the fish for 20 minutes, or until opaque in the middle.
Using a large spatula, transfer the fish to a warm platter along with the herbs, onion and lemon, and serve.