For the relish: Combine all ingredients in a medium bowl and season with salt and pepper to taste.
For the sauce: Preheat grill or side burner. Heat the oil in a large nonreactive saucepan over medium heat. Add the shallots and garlic and cook until soft. Add the Thai chiles, ginger, curry powder and cinnamon and cook for 2 minutes. Add the remaining ingredients and cook until the sauce becomes thick, about 30 to 40 minutes. Remove from heat and let cool slightly. Place plum mixture, in 2 or 3 batches, in a food processor and process until smooth. Remove marinade to a large bowl and let cool completely. Reserve 1 cup. Yield: 4 cups
For the duck: Preheat grill medium-low heat. Season breast with salt and pepper on both sides to taste. Grill skin-side down for 5 minutes. Baste, using 3 cups of the plum sauce, turn, baste again, and grill for 5 minutes. Baste, turn and baste again; cover the grill and cook the duck skin-side down for 5 minutes. Baste, turn and baste again; cover and cook another 5 minutes or until well-browned on the outside but medium-rare doneness (total 20 minutes). Remove breasts to a platter and brush with the reserved plum sauce. Cover with foil and let sit 10 minutes. Grill the tortillas for 15 seconds on each side. Cut the duck on the bias into 1/4-inch thick slices. Brush some of the reserved sauce in the center of each tortilla, place 3 or 4 slices of the duck and a few green onions, wrap and serve. Serve with Tomatillo Salsa.