Heat the oil in a large frying pan, add the garlic and tomatoes and cook over a medium heat, stirring, for 3–4 minutes.
Discard any clams that won't shut when tapped, then add them to the pan with the sherry and parsley. Season to taste with salt and pepper, then cover tightly and cook over a high heat, shaking the pan vigorously several times, for 4–5 minutes or until the clams have opened. Discard any that remain closed.
Serve the clams immediately in their cooking liquid or leave to cool to room temperature.