In a sauce pot, bring the clam juice to a boil. Add the fresh clams. Cover up and cook for 2 to 3 minutes until the clams open up. Remove the clams, set them aside and cover them up with plastic film wrap.
Melt the butter in a large, heavy sauté pan over low heat. Add the onions and garlic, stir well, cover, and cook about 10 minutes to allow the onions to soften. Uncover, and continue to cook, stirring frequently, until the onions are very soft but not browned, about 20 minutes.
Pour the clam juice slowly into the pan, add the bouquet garni, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Bring to a gentle simmer and cook, uncovered, 20 minutes. The liquid will begin to reduce and the chowder thickens. Add the cream and continue to cook very slowly to meld the flavors, another 10 minutes. Let cool slightly.
Transfer the chowder to a blender, add the truffle oil, and blend until very smooth. Pour the chowder through a fine-mesh sieve into a clean saucepan and reheat gently over low heat. The chowder should be fairly thick but still easily pourable. Taste and adjust the seasoning with salt and pepper. Display 5 clams in the half shell per warm soup plate. Divide and sprinkle the diced tomatoes among the plates. Ladle the chowder over the clams. Add a small chervil sprig or chives, and serve immediately.
Option: Serve with small toasted baguette croutons on the side.
The chowder comes by its velvety thickness simply by cooking. For the best result, take your time and make sure your onions do not brown.