Preheat the oven to 400°F (200°C). Rinse the whole fish under cold water and pat dry with paper towels. Season the cavity of the fish with salt and pepper, and place a few lemon slices inside.

Make the Broth

In a large oven-safe skillet or Dutch oven, heat the olive oil over medium heat. Add the chopped onion, bell pepper, celery, and minced garlic. Sauté until the vegetables are softened, about 5-7 minutes.

Add Tomatoes and Stock

Stir in the diced tomatoes and cook for another 2-3 minutes. Pour in the fish stock (or water) and bring to a simmer. Season with salt, black pepper, and chopped parsley.

Roast the Fish

Carefully place the whole fish on top of the broth in the skillet. Spoon some of the broth over the fish. Transfer the skillet to the preheated oven and roast for 25-30 minutes, or until the fish is cooked through and flakes easily with a fork.

Serve

Remove the fish from the oven and let it rest for a few minutes. Serve the fish with the broth and crusty bread for sopping up the delicious sauce.