For the salad: In a small bowl, combine the onions, celery leaves, and oil and toss well to coat. Season the salad, to taste, keeping cold until plating.
For the sauce: In a small saucepan over medium heat, cook the pomegranate juice, vinegar, and sugar for 10 to 12 minutes. Reduce the heat to medium-low, add the cinnamon, and cook for an additional 4 to 5 minutes, allowing the sauce to thicken to almost a syrup consistency. Once reduced, remove the sauce from the heat, whisk in the butter, allowing the butter to fully incorporate into the sauce. Hold at room temperature.
For the duck: Place the duck breasts on a cutting board, skin-side down, and slice from top to bottom into 6 or 7 (1/8-inch) thick slices. Season with salt and pepper on both sides, then thread each slice onto a 10-inch wooden skewer in a ribbon fashion. Heat the oil in a saute pan over medium-high heat until wisps of smoke appear. Season the duck again with salt and pepper and add to the hot pan, searing on each side for 2 minutes, cooking in batches if necessary.
To serve: Place 2 skewers of duck sate over a bed of salad and drizzle with the pomegranate sauce.