Clean the leeks well after chopping; sometimes they are sandy. In a 4-quart pot, warm the butter and oil over medium heat. Sauté the leeks and fennel in butter and olive oil until soft, about 5 minutes.
Add the potatoes, bay leaf, lemon pepper, salt, tarragon, clam juice and wine. Bring to a slow boil, simmer for approximately 20 minutes until the potatoes are fork tender.
Gently mix in the clams and half & half. Bring to a simmer.
Mix the cornstarch with 1/4 cup cool water to form a slurry. Slowly pour a little of the slurry into the simmering chowder, stirring constantly, to thicken the soup. Add just a little at a time until your desired thickness is reached. Do not use it all, if not needed. Serve warm!