If you cannot get the small Venus clams, use cockles, or small mussels, not the large clams which have a tendency to toughness and, in any case, are the wrong size for this dish.
Gently stew the onion, garlic and tomatoes in 3tbsp of the olive oil for 20–30 minutes until the onions have more or less disintegrated. Meanwhile, scrub the clams well under cold running water. Put them in a heavy saucepan with a lid, 4