In a small bowl, mix together raisins and apple cider vinegar. Let soak while you prepare the rest of the dish.
In a small pan over medium heat, toast walnuts for about 2-3 minutes, until they are golden brown. Let cool slightly, then chop.
Tear any lose outer pieces from radicchio. Cut in half, keeping the core in tact. Cut each half into 3-4 wedges, depending on size, keeping a sliver of core in tact for each so it stays together.
Heat 2 tablespoons olive oil in a cast iron pan over medium high heat. Add radicchio wedges, cut side down, and cook for about 2 minutes per side, until lightly charred. Transfer to plate.
In a small bowl, whisk together yogurt, 2 tablespoons olive oil, lemon juice, and a pinch of salt.
To plate, spread about half the yogurt sauce over the bottom of a platter. Place charred radicchio wedges on top, sprinkle with soaked raisins and walnuts. Drizzle yogurt dressing on top, then garnish with mint.
Recipe by Things I Made Today at https://www.thingsimadetoday.com/2017/11/16/charred-radicchio-yogurt-sauce-charleston-sc-trip-recap/