Bring 1 cm (½ inch) water to the boil in a saucepan, add the clams and steam until they open. Reserve the juice and remove half of the clams from their shells, keeping the remaining clams in their shells. Discard any clams that have not opened.
Heat the olive oil in a saucepan, add the chopped garlic and cook gently until golden but not brown. Add the courgettes, lemon rind and marjoram and turn them in the oil and garlic. Pour in the stock, season lightly with salt and pepper and bring to simmering point. Cover the pan and simmer for about 10 minutes or until the courgettes are soft.
Pass the soup through a coarse food mill and return it to the pan. Add the reserved clam juice and the shelled clams. If the soup is too thick, add extra stock or water. Taste and season with salt and pepper and a little lemon juice. Stir in the clams in their shells.
Rub the toasted bread with the bruised garlic clove. Place a slice in each bowl and ladle on the soup. Drizzle each serving with olive oil and serve immediately.