Preheat oven to 220°C.
Lightly score the skin of the fish; don’t cut too deep into the flesh.
Arrange fish and potatoes over two large baking trays. Season fish cavities with salt and pepper to taste, then fill with the tomatoes, half the lemon slices, and a little each of the garlic, rosemary, thyme and sage leaves. Scatter bay leaves and the remaining lemon slices, garlic, rosemary, thyme, and sage over the fish and potatoes. Drizzle with oil and season to taste with salt and pepper.
Roast fish until the skin is crisp and the flesh is just cooked through, about 20 minutes. The potatoes should be crispy-brown and cooked through. Allow to rest for 5 minutes before serving.