- 1 pound Yukon Gold potatoes, diced
- 3/4 cup chopped celery
- 1/2 cup chopped red bell pepper
- 4 ounces thick-sliced bacon, diced
- 1 1/2 cups chopped onion
- 2 tablespoons minced garlic
- 1 tablespoon fresh thyme leaves
- 1 3/4 teaspoons kosher salt, or to taste
- 1 teaspoon freshly ground black pepper
- 2 tablespoons all-purpose flour
- 4 cups clam juice, clam stock, or light fish stock
- 4 cups heavy cream
- 1 lb littleneck clams, scrubbed and purged 
- 1 pound mussels, scrubbed and debearded
- 1 pound skinless cod, halibut, or other firm white fish, cut into 1-inch cubes
- 1/4 cup sliced green onion
Place the diced potatoes in the bottom of the crock of a 6-quart slow cooker and sprinkle with the celery and bell pepper.
In a medium sauté pan, cook the bacon until it is crisp and most of the fat has rendered, 4 to 6 minutes. Add the onion, garlic, thyme, 1 1/2 teaspoons of the salt, and 3 /4 teaspoon of the pepper and cook until the onion is softened and the garlic is fragrant, 2 to 3 minutes. Add the flour and cook, stirring, for 2 minutes. Add the clam juice and cook, stirring, until the liquid comes to a boil and thickens, 2 to 3 minutes. Pour the hot bacon-stock mixture over the potatoes in the slow cooker.
Add the cream to the hot pan and bring to a simmer. Pour the hot cream into the slow cooker, cover, and cook, undisturbed, on high until the potatoes are tender, about 2 1/2 hours. Add the clams and cook until they have opened, about 30 minutes, stirring once midway. Add the mussels and cook until they have opened, about 15 minutes. Season the fish with the remaining 1/4 teaspoon salt and 1/4 teaspoon pepper and add to the slow cooker along with the green onion. Cook just until the fish is cooked through and flakes easily, about 15 minutes longer. Serve the chowder hot in shallow bowls.