Heat a little olive oil in a saucepan. When hot add the onion and garlic. When it softens add the stock, basil and chopped tomatoes. Turn the heat down so it simmers gently. Season with a little salt and pepper.
Prepare the olive paste. Put the olives and anchovies in a food processor with a little olive oil. Blend till smooth.
Remove the skin from the fish if it has it and any bones. Spread the fillets with the olive paste. Roll up and secure with cocktail sticks.
Gently place the fish into the sauce. Put the lid on the pan and leave to simmer for about 15 minutes.
Carefully remove the fish from the pan. Add the spinach and stir till it wilts then spoon the rest of the sauce over the fish.
Serve with fresh bread.
Notes
Nutritional information is approximate and a guideline only.