Pickled Carrots: Place hot water, sugar, salt and vinegar in a bowl. Whisk until sugar dissolved. Place carrots in pickling solution ensuring that carrots are submerged. Cover and refrigerate 4 hours to marinate. Drain and discard excess marinade.
For Service: In a small bowl, combine cucumber slices, pickled carrots and jalapeno slices. Mix well. Spread 1 tablespoon sriracha mayo on heel of bun. Place burger patty on heel of bun, top with grilled onions, vegetable mixture and cilantro sprigs. Spread 1 tablespoon yellow curry sauce on crown of bun. Close sandwich with crown of bun.