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.

Garnish clams with herbs and serve with crusty Italian or French bread.