Chinese Five-Spice Steak with Oranges and Sesame Broccolini

Finely grate 2 tsp. orange zest from 1 orange into a small bowl. Cut all oranges in half; set aside.
Add five-spice powder, brown sugar, and 2 tsp. kosher salt to bowl with zest and stir to combine. Rub steak all over with spice mixture.
Heat vegetable oil in a large heavy skillet (preferably cast iron) over high. Cook steak, turning often, until browned on all sides and an instant-read thermometer inserted into the center registers 130˚F for medium-rare, 15–20 minutes total. After steak has cooked for about 5 minutes, nestle orange halves cut side down around steak and cook until well browned, about 5 minutes. Transfer oranges to a plate. Transfer steak to a cutting board and let rest 10 minutes before slicing.
Arrange broccolini in an even layer in same skillet and heat over high. Cook, undisturbed, until well charred, about 3 minutes. Toss and continue to cook until tender and slightly charred all over, about 3 minutes more. Transfer to a large bowl. Squeeze in juice from 2 orange halves over. Add sesame oil, red pepper flakes, 1 tsp. sesame seeds, and remaining 1/4 tsp. kosher salt and toss to coat.
Transfer steak to a platter. Arrange broccolini mixture alongside. Top with scallions, sea salt, and remaining sesame seeds. Serve with charred orange halves to squeeze over, rice, and hot sauce alongside, if desired.