To make the cashew butter, pulse the cashews in a food processor until coarsely chopped. Add the butter, lemon juice, garlic, salt, and hot pepper sauce, and pulse until the cashews are finely chopped and the other ingredients are well blended. Transfer the butter to a small bowl, cover, and set it aside.
To make the potato salad, boil the potatoes in a large pot of lightly salted water until fork-tender, about 12 minutes. Drain.
Meanwhile, in a large bowl, whisk together the mustard and red wine vinegar. Add the hot potatoes. Toss to mix. Add the onions, celery, eggs, heavy cream, and parsley. Season with the salt, white pepper, and black pepper. Cover to keep warm.
Preheat the oven to 350?F.
Place a collapsible steamer in a large pot. Add water to reach just below the bottom of the steamer and bring to a boil over high heat. Add the spinach and cover tightly. Cook until the spinach is tender and wilted, about 5 minutes. Transfer the spinach to a colander and let cool completely.
Transfer the spinach, a handful at a time, to a bowl, squeezing the liquid from the spinach. Season with 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper.
Divide the spinach among six 15-inch squares of aluminum foil or parchment paper. Season the fillets with the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Using a flat metal spatula, smear both sides of each fillet liberally with the cashew butter. Lay a fillet on top of each portion of spinach, fold up to enclose the fillets, and tightly crimp the edges to seal the pouches.
Bake for 20 minutes.
To serve, transfer the papillotes to warmed dinner plates. Open at the table, using scissors. Serve the potato salad on the side.
Yield: 6 servings