12 tablespoons (1 1/2 sticks unsalted butter, divided
2 cups chopped yellow onions (2 onions)
2 cups medium-diced celery (4 stalks)
2 cups medium-diced carrots (6 carrots)
4 cups peeled medium-diced boiling potatoes (8 potatoes)
1 1/2 teaspoons fresh thyme leaves (1/2 teaspoon dried)
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1 quart (4 cups) clam juice
1/2 cup all-purpose flour
2 cups milk
3 cups chopped fresh chowder clams (1 1/2 pounds shucked clams)
Melt 4 tablespoon (1/2 stick) of the butter in a large heavy-bottomed stockpot. Add the onions and cook over medium-low heat for 10 minutes, or until translucent. Add the celery, carrots, potaotes, thyme, salt, and pepper and saute for 10 more minutes. Add the clam juice, bring to a boil, and simmer, uncovered, until the vegetables are tender, about 20 minutes.
In a small pot, melt the remaining 8 tablespoons of butter and whisk in the flour. Cook over very low heat for 3 minutes, stirring constantly. Whisk in a cup of the hot broth and then pour this mixture back into the cooked vegetabgles. Simmer for a few minutes until the broth is thickened.
Add the milk and clams and heat gently for a few minutes to cook the clams. Taste for salt and pepper. Serve hot.