Place a large pot or Dutch oven over medium-high heat and add the oil or butter. Once hot, add the diced onion and ground meat. Cook until the meat is browned and the onions are translucent. Stir in the minced garlic and cook for another 60 seconds until fragrant. Season the mixture with salt, pepper, and garlic powder to build a solid flavor base.

Add the rinsed rice directly into the pot with the meat. Stir it for 2 minutes to lightly toast the grains; this step is crucial for achieving that fluffy texture. Pour in the broth and add the frozen vegetables. Bring the liquid to a rolling boil, then immediately reduce the heat to low. Cover the pot with a tight-fitting lid and let it simmer undisturbed for 18 to 20 minutes.

Once the liquid is fully absorbed, turn off the heat. Do not open the lid yet! Let the pot sit covered for 5 minutes to steam the rice to perfection. Remove the lid, sprinkle the shredded cheese over the top, and cover again for 2 minutes to let the cheese melt. Fluff the entire casserole gently with a fork and serve hot straight from the pot.