Heat one tablespoon of oil in a large skillet over medium-high heat. Add the chicken pieces and cook for about 5 minutes, stirring occasionally, until they're nicely browned on the outside. Remove the chicken from the pan and set it aside on a plate.
Add the remaining tablespoon of oil to the same skillet. Toss in the chopped onion and garlic, cooking for around 4 minutes until they turn soft and fragrant. Scrape up any browned bits stuck to the bottom of the pan, since that's where a lot of flavor hides.
Pour in the water and bring it to a boil, then stir in the yellow rice mix. Once it returns to a boil, reduce the heat to low, cover the skillet, and let it simmer for about 20 minutes. This gives the rice time to soften and soak up all that oniony flavor.
Once the rice has simmered, layer the broccoli florets and browned chicken right on top. Cover the pan again and cook for another 5 minutes, just until the broccoli turns bright green and everything is heated through.
Sprinkle the shredded cheddar evenly over the top of the skillet. Cover it, remove the pan from the heat, and let it sit for about 5 minutes. The residual heat melts the cheese into gooey perfection without overcooking anything underneath.