Preheat the duck fat to 260 to 270 degrees F. Preheat the oven to 300 degrees F. Preheat the grill over medium-high heat.
Add the potato slices to the hot oil and blanch for about 1 minute. Strain onto a baking sheet. Spread them out to cool evenly and let sit until cool enough to handle.
Place a large piece of plastic wrap out on work surface. Overlap the blanched potatoes creating a bridge connecting the pieces together making it big enough to fit 2 pieces of the Pacific cod. Season the fish fillets with salt on both sides and add to the potatoes perpendicular to the direction of the potatoes. Using the plastic wrap, fold and tuck the potatoes around the fish. Wrap the whole piece with the plastic wrap to bond.
Place a large nonstick pan over medium heat and add 1/2 cup duck fat. Once heated, add the fish, in batches if needed, seam-side down and season with salt. Let it brown for about 2 minutes then flip. Add the thyme and butter to the pan. Allow the butter to melt and baste fish to add more flavor and moisture to the fish and potatoes. Place the whole pan into the oven for 4 to 5 minutes to cook through.
Drizzle extra-virgin olive oil on the cleaned leeks, sliced potatoes and the split lobster tail. Season with salt and place onto the grill and cook until tender, about 4 to 5 minutes.
For the vanilla bean and tarragon vinaigrette: To a bowl, add the vanilla seeds, juice of 1 blood orange, minced shallots, pinch salt, splash champagne vinegar and whisk in about 1/2 cup extra-virgin olive oil so its 2 parts oil, 1 part vinegar.
Remove the lobster from the shell and chop. Chop the potatoes and leeks and add all to the vinaigrette bowl. Add the tarragon and season with salt and freshly cracked black pepper and gently toss to combine.
Remove the fish from the oven and place onto a clean surface. Slice in half. Place a large spoonful of the potato salad onto the plate. Top with 1 piece of fish and drizzle with some of the dressing. Season with a pinch of sea salt.
You can place the wrapped fish into the refrigerator prior to cooking to help it set up more.