Preheat your oven to 350°F.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned, about 5-7 minutes. Drain any excess fat.
Stir in the sliced mushrooms and minced garlic. Cook for an additional 3-4 minutes until the mushrooms are tender.
Season the mixture with salt, black pepper, and Italian seasoning. Stir in the drained diced tomatoes and cooked rice until well combined.
Transfer the mixture to a greased 9x13-inch baking dish. Sprinkle the shredded cheddar cheese evenly over the top.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.