Step 1Heat the oven to 300 degrees. Peel the potatoes, cut them into small cubes, place in a saucepan, cover with cold water, heat to a boil and cook until tender, about 20 minutes. Drain the potatoes, place in a baking dish or pan and set in the oven to dry out and stay warm, 10 to 15 minutes.
Step 2Heat 3 tablespoons of butter in a skillet over medium heat. Add the onion, garlic and shallots and cook, stirring frequently, until the onion is translucent, 7 to 9 minutes.
Step 3Raise the heat, add the wine and cook, stirring occasionally, until the liquid has evaporated, about 5 minutes. Add the sausage and cook, stirring frequently, 5 minutes.
Step 4Combine with the potatoes in a large bowl, season generously with salt and pepper, and mash with a potato masher until the potatoes are fairly smooth. Cover and keep warm.
Step 5Combine the lemon juice, zest and 2 teaspoons of water in a saucepan over moderately high heat and bring to a boil. Lower the heat and whisk in the remaining 4 tablespoons butter, 1 tablespoon at a time, until all the butter is incorporated and the sauce coats the back of a spoon. Remove from heat and cover.
Step 6Rinse and pat dry the cod fillets and season on both sides with salt and pepper. Heat the olive oil in a large nonstick skillet over high heat, add the fillets, and cook until one side is crisp and browned, 3 to 4 minutes. Turn the fillets and cook until just cooked through, about 3 minutes. Remove the bones.
Step 7To serve, place a mound of the potatoes and sausage in the center of six plates, flatten each with a spatula, top with a cod fillet, browned side up, sprinkle with chives and drizzle with lemon butter sauce.