Melt the butter in a large enameled cast-iron casserole. Add the bacon and cook over moderately high heat, stirring, until lightly browned, about 2 minutes. Add the onion and cook, stirring occasionally, until softened, about 7 minutes. Stir in the garlic and crushed red pepper and cook, stirring, until the garlic is fragrant, about 2 minutes.

Add the clam broth, chicken stock and bouquet garni to the casserole and bring to a boil over high heat. Lower the heat to moderately high heat and simmer uncovered for 20 minutes.

Add the diced potatoes to the soup and cook over moderately high heat until just tender, about 10 minutes. Discard the bouquet garni.

In a medium bowl, whisk 1/4 cup of the heavy cream with the cornstarch until smooth. Whisk in the remaining 2 cups cream, then whisk the mixture into the soup. Bring the soup to a boil over moderately high heat. Add the leeks and cook until just tender, about 4 minutes.

Stir the scallops into the chowder and cook over moderate heat just until opaque throughout, 2 to 3 minutes; don't let the soup boil. Season with salt and pepper. Ladle the chowder into a tureen or individual bowls. Garnish with the chopped chives and serve at once.