Cover a cutting board completely with a piece of plastic wrap. Arrange pork medallions in a single layer on the plastic wrap-covered cutting board. Cover with a second piece of plastic wrap. Pound medallions to an even thickness, about 1/4-inch. Remove top layer of plastic wrap and season to taste with salt and pepper (I like 1/2 teaspoon salt and 1/4 teaspoon pepper).
In a shallow dish, add 3 tablespoons maple syrup. In a second dish, add ground walnuts. Working with one medallion at a time, coat pork all over in syrup, then dredge in walnuts. Press down all over to make sure walnuts adhere well. Set aside and repeat with remaining medallions.
In a large skillet over medium-high heat, heat oil until shimmering. Working in batches if necessary, place walnut-crusted pork in a single layer. Cook until browned on one side, about 3 to 4 minutes, then flip. Continue cooking until pork reaches 145 degrees when tested with an internal thermometer, about 2 to 3 minutes longer. Transfer to a serving platter.
To the same skillet, stir in orange marmalade, water, 1 teaspoon apple cider vinegar, remaining 1 tablespoon syrup, cumin, and cayenne. Cook, uncovered, until slightly thickened, about 1 to 2 minutes. Season to taste with salt and pepper, and add another teaspoon of apple cider vinegar if the flavor is too sweet. Pour over pork. Serve with orange wedges.