How to Make It
In a large skillet, bring 1/4 inch of water to a boil. Add the clams, cover and cook until opened, about 3 minutes. Transfer the clams to a baking sheet to cool. Strain the cooking liquid through a fine sieve into a medium bowl. When the clams are cool enough to handle, remove them from their shells and pull off the brown outer membrane. Rinse each clam in the cooking liquid, then strain the cooking liquid again through a fine sieve to remove any sand or grit.
Put the potatoes in a saucepan of cold salted water and bring to a boil. Simmer until just tender, about 5 minutes; drain.
In a blender, puree the chives with 1/3 cup of olive oil until smooth. Using a small spatula, press the puree through a fine sieve into a small bowl. Season with salt.
In a small saucepan, combine the ground pistachios with 1/2 cup of the reserved clam cooking liquid and bring to a boil. Remove from the heat and whisk in 3 tablespoons of the butter. Stir in the lemon juice and season the sauce with salt and pepper. Keep warm.
Season the cod with salt and pepper and dust with flour. Heat the remaining 3 tablespoons of olive oil in a large skillet over moderately high heat. Add the cod, skinned side up, and cook until golden, about 4 minutes. Turn the cod and cook for 3 to 4 minutes longer, or until just opaque throughout. Transfer the cod to a plate and keep warm.
In a medium skillet, heat 2 tablespoons of the clam cooking liquid. Add the clams, potatoes and the remaining 1 teaspoon of butter and simmer just until heated through. Pour the pistachio sauce onto plates. Divide the clams and potatoes among the plates and set the cod on top. Drizzle the plates with the chive oil and serve.