Set a 12-inch saute pan over medium heat and cook the bacon strips until crisp, about 8 to 10 minutes. Transfer to paper towels and set aside.

Place the potatoes in a 1-quart saucepan and cover with water. Bring to a boil and just until tender, about 8 to 10 minutes. Refresh under cold running water, and set aside.

Preheat the oven to 400 degrees F.

Melt the butter in a large, heavy saucepan over medium-high heat. Add the chopped bacon until it begins to brown, about 5 minutes. Add the leeks and onions and cook, stirring, until soft, about 4 minutes. Add the garlic and cook, stirring, for 30 seconds. Add the flour and cook, stirring constantly, until a light blond roux forms, about 5 minutes. Add the cream, shrimp stock, and 1/2 teaspoon each of the salt and white pepper and bring to a simmer. Simmer for 10 minutes, stirring occasionally. Add the shrimp, clams and potatoes and stir well. Reduce the heat to medium-low, cover, and simmer, stirring occasionally, until the clams open and the shrimp are just cooked through, 6 to 8 minutes. Remove from the heat and discard any clams that did not open. Cover to keep warm.

Heat the olive oil in a large nonstick ovenproof skillet, or in 2 smaller skillets, over high heat. Season each halibut fillet on both sides with 1/8 teaspoon each of the remaining salt and white pepper. Place the fillets in the pan(s), reduce the heat to medium-low, and cook until a golden brown crust forms, about 3 minutes. Turn the fish, place in the oven, and roast until just cooked through, 4 to 5 minutes. Remove from the oven.

To serve, ladle the chowder into 6 rimmed soup bowls, arranging 3 clams around the rim of each bowl. Place a halibut fillet in the center, and lay 3 strips of bacon on top of the fish. Garnish with the parsley and serve immediately.