For the fish: Line a quarter sheet pan with parchment paper. Butter the parchment with 2 tablespoons of the butter, then smear another 2 tablespoons on the halibut fillets. Season the fillets on both sides with salt and place in the oven to slow roast until opaque and cooked through, about 15 minutes.
Meanwhile, heat a deep-sided pan over medium-high heat until very hot. Add the remaining 8 tablespoons butter, the clams, garlic, shallots, parsley and a pinch of salt. And the wine, immediately cover with a lid and steam until the clams open, about 5 minutes. Transfer the clams to a bowl and set aside. Pour the clam cooking liquid through a fine-mesh strainer and set aside.
For the beurre blanc: Combine the reserved clam liquid, Champagne, thyme, bay leaf, shallots, lemon juice and peel and peppercorns in a large saucepan. Bring to a boil over medium-high heat and reduced by two-thirds (the saucepan will be almost dry), 10 to 15 minutes. Strain through a fine-mesh strainer fitted with a coffee filter into a small pot.
Place the pot over medium-low heat, add a few cubes of the chilled butter and whisk constantly until the butter is melted. Repeat the process 2 more times with the remaining butter. Don’t allow the sauce to boil or it will break. Whisk in the creme fraiche and a pinch of salt. Using an immersion blender, blend the uni into the sauce, then stir in the chives. Remove the clams from their shells, add them to the beurre blanc and keep the sauce warm until ready to serve.
For serving: Cover the bottom of each plate with a layer of zucchini slices, slightly overlapping them in a circular pattern. Top with the lemon zest and a drizzle of olive oil and season with salt and pepper. Place a halibut fillet on the zucchini, spoon the beurre blanc over the fish, then top with caviar.