Preheat your oven to 350°F (175°C). Rinse the pork under cold water and pat it dry thoroughly with paper towels. Using a sharp knife, make deep crosswise cuts along the length of the pork, spacing them about 0.4 inches (1 cm) apart. Cut deeply but stop short of slicing all the way through—the base should stay connected to hold the stuffing. Season both sides generously with salt and pepper, rubbing the spices into the meat.
Melt the butter in a skillet over medium-low heat. Add the chopped onion and sauté for 7-10 minutes, stirring occasionally, until soft, translucent, and lightly golden. Set aside to cool slightly.
While the onions cook, coarsely chop the walnuts.
Slice the pitted plums or prunes into thin wedges. (If using fresh plums, choose ripe but firm ones for the best texture.)
Gently open each cut in the pork and stuff them evenly with the plum or prune slices, pressing them in snugly. Spread the sautéed onions evenly over the top of the pork, followed by a generous layer of chopped walnuts, pressing lightly so they adhere.
Transfer the pork to a baking dish. Cover it tightly with aluminum foil, sealing the edges to trap the moisture and flavors. Place in the preheated oven and bake for 50-60 minutes, or until the pork is fully cooked—juices should run clear when pierced, and the internal temperature should reach 145°F for medium doneness.
Remove the pork from the oven, carefully open the foil, and let it rest for 15-20 minutes to lock in the juices. For a golden, crispy walnut topping, return the uncovered pork to the oven for an additional 15 minutes, keeping an eye on it to prevent burning.
Slice the pork between the cuts to reveal the stuffed layers. Serve warm, spooning any pan juices over the top. Pair it with roasted vegetables, mashed potatoes, or a simple green salad for a complete meal.