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