Cut the fish fillets into bite-sized pieces, sprinkle with lemon juice, and season with salt and pepper.
Peel the onion and chop finely. Peel the garlic and cut into thin slices. Rinse the tomatoes and cut into small pieces.
Heat olive oil in a pan and saute the onions until soft, do not brown. Add the garlic and tomatoes, and briefly fry. Pour in the stock and simmer for 5 minutes. Season with salt and pepper. Add the fish pieces and olives, cover the pan and cook for another 5 minutes.
To serve, arrange the stew on plates and garnish with some Parmesan shavings.