Heat 2 tablespoons of olive oil in a deep skillet over medium heat. Add the chicken and season with the garlic powder, onion powder, celery salt, and black pepper. Cook for about 5 minutes, stirring occasionally, until lightly browned. Transfer the chicken to a plate and set aside—it will finish cooking later.
Add the butter to the same skillet. Once melted, stir in the rice and toast for about 3 minutes, stirring occasionally, until lightly golden and nutty. Transfer the rice to a bowl and set aside. Don’t worry if a few grains remain in the pan.
Add the remaining tablespoon of olive oil. Sauté the onion for 4-5 minutes, until softened and translucent. Add the garlic and cook for another minute, until fragrant.
Return the toasted rice to the skillet and pour in the chicken stock. Stir well and bring to a bubble. Reduce the heat to low, cover, and simmer for about 12 minutes.
Stir the rice, then add the chicken and broccoli. Mix everything together, adding a splash of the additional stock if the skillet looks too dry. Cover, increase the heat to medium-low, and cook for another 4-5 minutes, until the broccoli is tender, the rice is fluffy, and the chicken is fully cooked.
Remove the pan from the heat and add the dijon mustard and most of the shredded cheese, stirring to combine. Taste and adjust seasonings if needed.
Finish with the rest of the cheese on top and place the lid on one more time to let the top layer of cheese melt for a few minutes. Serve while it’s warm and melty!