Dissolve 1/4 cup salt in 1 cup cold water in a medium bowl. Add the pork medallions, cover and refrigerate 1 to 4 hours.
Meanwhile, spread the fruitcake on a plate and microwave until slightly dry, 1 to 2 minutes. Chop any large pieces of fruit or nuts, then let cool. Mix the butter, mustard, Worcestershire sauce, 1/2 teaspoon salt, and pepper to taste in a medium bowl. Mix in the dried fruitcake and refrigerate until firm, about 30 minutes.
Drain the pork and pat dry. Insert a paring knife into the curved side of each medallion to create a small pocket in the middle. Open the pocket with the knife or your fingertip and tuck spoonfuls of the fruitcake stuffing inside. Secure each opening with a toothpick.
Preheat the oven to 350 degrees. Season the pork with pepper. Heat the vegetable oil in a large skillet over high heat, then add the pork and cook until golden brown, about 3 minutes per side. Transfer to a baking dish and bake until cooked through, about 12 minutes.
Wipe out the skillet. Add the sherry, then return to medium heat, bring to a boil and reduce by about half. Add the cream and simmer until thickened, about 5 minutes. Season with salt and pepper. Remove the toothpicks and slice the pork medallions in half crosswise. Serve with spinach, if desired, and drizzle with the cream sauce.