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.
Ready in 20 min.