Wine-Steamed Clams

1 medium onion, chopped
2 small garlic cloves, sliced thin
1 tablespoon olive oil
1/2 cup dry white wine or dry vermouth
2 dozen small hard-shelled clams, scrubbed well
1 cup fresh coriander sprigs, washed and spun dry
1/2 cup fresh flat-leafed parsley leaves, washed and spun dry
1 tablespoon unsalted butter
1 tablespoon fresh lemon juice, or to taste
Fresh coriander and/or parsley sprigs (for garnish)

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.