- Season chicken liberally with salt and pepper.
- Using hot grill with an oiled grate, place chicken on grate, skin side up. Cover grill and cook until skin begins to render and firm up, about 5 minutes.
- Gently turn chicken over and brush liberally with agrodolce. Cover grill and cook chicken skin side down another 5 minutes.
- Turn once more to skin side up, brush again with agrodolce, and cook through, about 2-5 minutes more, until internal temperature is 160 degrees. Brush again with agrodolce and remove from grill.
- Cut chicken into large chunks, lay on plate with arugula, pine nuts, and pickled grapes. Drizzle with oil and serve immediately.
- Combine sugar, vinegars, and mirin in saucepot and bring to a boil. Add chilis and garlic and return to a boil.
- Cover and refrigerate overnight.
- Using blender, stick blender, or food processor, purée to chunky consistency. Keep refrigerated.
- Combine water, vinegar, and sugar in saucepot and bring to a boil.
- Pour boiling liquid over grapes.
- Refrigerate. Grapes are ready to use when fully cooled.