Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, cook the ground beef until browned, breaking it apart with a spoon, about 5-7 minutes.
Add the diced onion, minced garlic, diced green bell pepper, and sliced mushrooms to the skillet. Cook until the vegetables are softened, about 5 minutes.
Stir in the diced tomatoes with their juice, tomato sauce, dried Italian seasoning, salt, and black pepper. Simmer the mixture for 5 minutes, stirring occasionally.
Remove the skillet from heat and stir in the cooked white rice until well combined.
Transfer the mixture to a 9×13-inch baking dish and spread evenly.
Sprinkle the shredded cheddar cheese evenly over the top.
Cover the baking dish tightly with aluminum foil and refrigerate if making ahead. When ready to serve, bake covered for 30 minutes.
Remove the foil and bake uncovered for an additional 10 minutes, or until the cheese is melted and bubbly.
Let the dish rest for 5 minutes before serving.