For the sticky rice, rinse the Arborio rice several times; drain. Add to a medium-sized pot with 2 cups of water and bring to a boil. Reduce the heat and simmer, covered for 15 minutes. Remove from the heat and let sit, covered, for another 15 minutes (or until ready to serve). Fluff with a fork and season with salt. Garnish with black sesame seeds if desired.
For the steak, in a large bowl combine the soy sauce, sesame oil, rice wine vinegar, Sriracha, honey, garlic, ginger and oil in a large bowl. Pour half of the sauce into a smaller bowl and set aside. Add the flank steak into the large bowl of marinade and let sit for 10 minutes.
For the cucumber salad, in a large bowl whisk together the rice wine vinegar, soy sauce, Sriracha, sesame oil, sugar, garlic, and red pepper flakes. Stir in the cucumbers and season to taste with salt and pepper (if needed).
Heat a grill pan over high heat. Remove the steak from the marinade and season each side with salt and pepper; discard the marinade.
Grill for 5-6 minutes per side (10-12 minutes total) for medium-rare. Remove to a cutting board with grooves around the edge to catch juices and let sit for 5 minutes. Thinly slice the meat across the grain. Pour the meat juices into the small bowl of remaining sauce, stirring to combine.
Serve the steak, drizzled with sauce and sprinkled with green onions, with the sticky rice and cucumber salad.