Step 1Heat the oven to 325 degrees. Remove the stems from the grapes and wash them in cold water. Place them in a medium saucepan with 1 teaspoon salt, white pepper and 1 tablespoon of olive oil. Add enough water to cover the grapes. Cook over medium heat, covered, for 30 to 40 minutes, until the grapes are thoroughly cooked and softened.
Step 2Using a blender or immersion blender, blend the mixture until it is a smooth sauce and the grape skins are completely dissolved. If the skins do not completely break down, return the pan to the stove and cook a few additional minutes until they dissolve when blended. Check and adjust the seasoning if needed. Set aside in a warm place.
Step 3While the grapes are cooking, cut the chicken in half using kitchen shears and remove and discard the backbone. Heat a grill over high heat. Season each chicken half with three-fourths teaspoon salt and a pinch of black pepper, and rub with the lemon zest, sage, garlic and red chile flakes. Drizzle lightly with the remaining tablespoon of olive oil and place on a hot grill.
Step 4Sear the chicken for 3 minutes on each side to brown. Remove the chicken from the grill and place it on a baking sheet. Place the baking sheet in the oven and roast 30 to 40 minutes until done; the meat will be firm, and a thermometer inserted will read 165 degrees.
Step 5Remove the chicken to two plates and drizzle with the warm grape sauce. Garnish each half with the peeled and seeded grapes. Serve immediately.