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.
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.
Place 2 skewers of duck sate over a bed of salad and drizzle with the pomegranate sauce.
