In a bowl, mix the herbs with the garlic and olive oil. Add the lemon zest and juice and stir gently. Refrigerate until ready to serve.
Meanwhile, preheat the oven to 500°. Grease a baking sheet with olive oil.
Rinse the fish and pat dry. Trim off the fins.
Scatter the herb stems, bay leaves and garlic on the baking sheet.
Place a row of lemon slices down the center of pan; reserve one slice. Place half of the onion slices over the lemon slices.
Place the fish over the onions and season liberally with salt and pepper. Place the remaining onions on top of fish.
Drizzle with olive oil and place the remaining slice of lemon on top of fish. Cover with foil and bake until the fish is cooked through, about 20 minutes. Serve the fish whole, with the salmoriglio sauce on the side.