12 littleneck or manilla clams, or mussels
2 tablespoons olive oil
2 slices Canadian bacon (2 ounces total;optional), diced
3 ribs celery, diced
2 medium carrots, scrubbed and sliced 1/4-inch thick
1 medium yellow onion (8 ounces), diced
2 garlic cloves, minced or finely grated
2 tablespoons tomato paste, no-salt-added, if desired
12 ounces waxy potatoes, such as yellow creamer, fingerling or new, cut into 1/2-inch pieces
One (14-ounce) can no-salt-added diced tomatoes, with their juices
2 cups low-sodium fish or seafood stock
3 sprigs fresh thyme or 1 teaspoon dried thyme leaves
1 bay leaf
1/4 teaspoon fine salt, plus more to taste
1/4 teaspoon crushed red pepper flakes
1 1/2 pounds skinless firm white fish fillets, such as cod, sea bass or hake, cut into 1 1/2-inch chunks
8 ounces (20/30) scallops or peeled and deveined large (20/30) shrimp (tails on or off)
3 tablespoons chopped fresh flat-leaf parsley leaves

Large Dutch Oven

1. Soak the clams in a deep bowl of cold water for about 1 hour, so they can discharge any grit.
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2. In a large (7to 8-quart) Dutch oven or heavy-bottomed pot over medium-high heat, heat the oil until shimmering. Add the bacon, if using, and cook, stirring occasionally, until browned, about 2 minutes. Using a slotted spoon, transfer the bacon to a small plate. Reduce the heat to medium, then add the celery, carrots and onion, and cook, stirring occasionally, until just softened, about 5 minutes. Stir in the garlic and cook until aromatic, 30 seconds to 1 minute, then add the tomato paste and cook, stirring frequently, until darkened, about 1 minute more.
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3. Add the potatoes, tomatoes with their juices, fish stock, thyme, bay leaf, salt and crushed red pepper flakes. Bring to a boil, then reduce the heat to medium-low and cook, uncovered, stirring occasionally, until the potatoes are tender, about 25 minutes. Discard the bay leaf and thyme stems, and stir in the Canadian bacon, if using. Taste, and season with additional salt if desired.
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4. Drain the clams, discarding any that do not close when gently tapped. Add the clams to the pot, cover, and cook until they begin to open, about 5 minutes. Add the fish and scallops or shrimp, and stir gently to combine. Increase the heat to medium-high and return the stew to a boil. Reduce the heat to medium-low, cover and cook, stirring once or twice, until the seafood is no longer translucent, the fish flakes easily with a fork and the clams have opened, about 5 minutes more. (Discard any clams that do not open.) Divide among individual shallow bowls, garnish with the parsley and serve hot.
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Substitutions: For yellow onion, use white onion or shallots. For Canadian bacon, use regular bacon or pancetta.