Season steak with salt and generous amount of dry rub on one side, and salt & pepper on the other side. Let marinate for 30 minutes while the meat comes up to room temperature. Cut the onions in thick rounds and season with salt and dry rub on both sides and put aside. In a small pot, combine the water, sugar, and vinegar and bring to a boil. Transfer the vinegar mixture to a bowl and cool. Add the carrot and daikon and let marinate for 30 minutes or put in the fridge overnight. For the Sriracha mayo, Combine the mayo, sriracha, sesame oil, and lime juice in a small bowl and stir to combine. Will keep in the fridge for up to a week. Pre-heat a grill pan or BBQ grill to high. For the grill pan, cook meat for 4-5 minutes each side for medium rare and let meat rest under foil for 5 minutes. For BBQ grill, cook meat over high heat with the lid open for 3-5 minutes, flip and put the lid down for the last 3-4 minutes of cooking. Let meat rest. For the onions, cook 5-10 minutes each side until the onions have nice grill marks and have softened. To assemble the sandwich, toast the bread on the grill and cut the steak in slices, along with the onions. Give a generous schmear of the mayo on the bread, add the steak, onions, carrots, daikon, and cilantro. Roll the cucumber spears in a 50/50 mixture of chili powder and sugar, and serve as garnish.