Preheat oven to 400°F. Combine chicken pieces, 1 tablespoon Sriracha and 2 tablespoons mayonnaise in a medium bowl; stir until well coated. Sprinkle with 2 tablespoons flour; stir until combined.

Heat 2 tablespoons oil in a large skillet over medium-high heat until shimmering. Add the chicken; cook, undisturbed, until golden brown on the bottom, about 3 minutes. Turn the chicken and cook until golden brown on the other side, 3 more minutes. Sprinkle with the remaining 2 tablespoons flour; cook, stirring constantly, until fragrant, about 30 seconds. Remove from heat; stir in ¾ cup broth, scraping up any browned bits from the bottom of the pan.

In a large bowl, combine chopped scallions, cream cheese pieces, ¼ cup chili sauce, the minced garlic, 1 tablespoon each onion powder and vinegar, ½ teaspoon salt and the remaining 2 tablespoons Sriracha; stir until well blended. Add 6 cups broccoli, 1 package rice and the chicken mixture; stir until thoroughly combined.

In a small bowl, stir together ⅓ cup panko and the remaining 1½ teaspoons oil.

Transfer the chicken mixture to a 9-by-13-inch baking dish; spread in an even layer. Sprinkle with the panko mixture. Bake until the broccoli is tender, the sauce has thickened slightly and an instant-read thermometer inserted into the thickest portion of chicken registers 165°F, 20 to 25 minutes. Let stand for 5 minutes before serving; garnish with scallions, if desired.