Gather all ingredients. Preheat the oven to 375 degrees F (190 degrees C).

Toss together chicken, onion powder, 2 tablespoons Sriracha, 2 tablespoons soy sauce, and 1 tablespoon garlic until well combined in a large bowl. Sprinkle with flour until completely coated.

Heat oil in a large cast-iron skillet over medium-high heat until shimmering. Working in 2 batches, add chicken in an even layer; cook, undisturbed, until browned on both sides, about 6 minutes total, flipping halfway through. Transfer chicken to a plate. Do not wipe skillet clean. Repeat with remaining chicken.

Reduce heat to medium and stir in rice, 2 tablespoons sweet chili sauce, 1 ½ teaspoons rice vinegar, remaining 1 tablespoon soy sauce, and remaining 1 tablespoon garlic. Cook, stirring occasionally, until fragrant and rice is toasted, about 2 minutes.

Stir in broth and bring to a boil over medium-high. Remove from heat and top evenly with chicken. Cover with a tight-fitting lid or aluminum foil.

Bake in the preheated oven until rice is tender and cooked through, 22 to 25 minutes.

Meanwhile, whisk together mayonnaise, remaining ¼ cup sweet chili sauce, 2 tablespoons Sriracha, and 1 ½ teaspoons rice vinegar until smooth. Transfer to a squeeze bottle, or transfer to a resealable plastic bag, reseal the bag, and cut off a small corner.

Drizzle casserole with bang bang sauce as desired. Garnish with scallions and sesame seeds.