In a large heavy saucepan cook onion and half of garlic in oil over moderately
low heat, stirring, until softened.
Add wine or vermouth and simmer, uncovered, stirring occasionally, 5 minutes.
Add clams and steam, covered, over moderately high heat 5 to 7 minutes,
transferring them as they open with tongs to a serving bowl and reserving cooking
liquid. (Discard clams that are unopened after 7 minutes.) Keep clams warm.
In a blender blend together coriander, parsley, butter, remaining garlic,
lemon juice, and reserve cooking liquid until smooth.
Pour sauce over clams and toss to combine.
Garnish clams with herbs and serve with crusty Italian or French bread.