Wash clams thoroughly several times and soak in cold water to remove trace sand.
While clams are soaking, prepare broth.
In a large deep skillet sauté onion in olive oil for about 3 minutes until onions are soft.
Add the pancetta and brown for 2 to 3 minutes.
Add the sliced garlic and continue to sauté for about 2 minutes until just golden.
Add the remaining ingredients, except tarragon and clams, and bring to a boil over high heat.
Decrease heat to low and reduce liquid by one third, about 5 minutes.
Add washed clams. Simmer covered, over low heat for about 5 minutes until clams open.
Remove from heat and add the tarragon.