Start by heating your large skillet over medium-high heat without adding oil. Crumble the ground beef directly into the hot pan, breaking it apart with your wooden spoon. Cook the beef for 6-8 minutes, stirring frequently to ensure even browning and to prevent sticking. Once the meat is completely browned with no pink remaining, drain the excess grease by tilting the pan and carefully spooning out the fat.
Return the drained beef to the skillet and reduce heat to medium-low. Pour in the cream of mushroom soup, stirring well to coat all the meat. Add the diced tomatoes with their juice, followed by the uncooked rice, mixing everything thoroughly. The mixture will look quite liquidy at this stage, which is exactly what you want for the rice to cook properly.
Cover the skillet with a tight-fitting lid and let the mixture simmer on low heat for about 15 minutes. Stir occasionally to prevent the bottom from burning, but don't lift the lid too frequently as this releases steam needed for cooking the rice. The casserole is ready when the rice is tender and has absorbed most of the liquid, creating a creamy, cohesive mixture.
Preheat your oven's broiler while the casserole finishes cooking. Transfer the mixture to your prepared casserole dish, spreading it evenly. Sprinkle the mozzarella cheese generously over the top, ensuring complete coverage. Place under the broiler for 2-3 minutes until the cheese is melted, bubbly, and golden brown on top.