Preheat oven to 450 degrees. Arrange racks in upper and lower thirds of oven.
Cut each yam lengthwise into six spears. Place on baking sheet, and toss with brown sugar, 1 tablespoon oil, and 1/4 teaspoon each salt and pepper. Place on lower rack of oven, and roast 15 minutes. Remove from oven, and turn over each spear with a metal spatula. Return to lower rack of oven until well browned, about 15 minutes. Remove from oven, and keep warm until ready to serve.
After returning yams to oven, combine breadcrumbs, oregano, and remaining 1/4 teaspoon each salt and pepper in a shallow bowl. Dredge pork medallions one at a time in breadcrumbs, turning to completely coat. Transfer to a plate while repeating with remaining medallions.
Coat a shallow baking dish or cast-iron skillet with remaining 1/2 tablespoon oil. Add pork medallions, and transfer to upper rack of oven. Cook 6 minutes; turn and cook until pork is browned and firm to the touch, about 6 minutes more. Transfer to a cutting board.
While pork and potatoes are cooking, place wet spinach leaves in a large saucepan over high heat. Cover, and steam, stirring occasionally, until wilted, about 3 minutes. Remove from heat.
Slice pork on the bias, and place on serving plate with yams, steamed spinach, and lime wedges.