Heat the oil in a large heavy-bottomed pot or Dutch oven over medium-high heat until shimmering. Meanwhile, toss the chicken with 1 teaspoon of the seasoning. Add the chicken to the pot in a single layer and cook undisturbed until browned on the bottom, 4 to 5 minutes. Using a slotted spoon, transfer to a plate. (Chicken will not be cooked through at this point.)

Add the onion, celery, and bell pepper to the pot and cook, stirring occasionally, until softened, about 6 minutes. Add the rice, garlic, salt, black pepper, and remaining 1 ½ teaspoons seasoning. Cook, stirring occasionally, until the rice is starting to toast and smell fragrant, 1 to 2 minutes.

Add the milk and broth or water and bring to a simmer, stirring occasionally so that the rice doesn't stick to the bottom. Return the chicken and any accumulated juices to the pot, stir to combine, and bring back to a simmer.

Reduce the heat to medium-low, cover, and simmer until the liquid is absorbed and the rice is tender, 15 to 20 minutes. Remove from the heat, sprinkle with the scallions, and let sit covered for 5 minutes before serving.