Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, cook the ground beef until browned and no longer pink, breaking it up with a spoon as it cooks, about 6-8 minutes.
Add the diced onion and minced garlic to the skillet and cook until the onion is translucent, about 3-4 minutes.
Stir in the diced bell pepper and sliced mushrooms, cooking for another 4-5 minutes until vegetables are tender.
Add the canned diced tomatoes with their juice, tomato sauce, dried Italian seasoning, salt, black pepper, and crushed red pepper flakes. Stir to combine and simmer for 5 minutes.
Remove the skillet from heat and stir in the cooked rice until evenly mixed.
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 with aluminum foil and refrigerate if making ahead, up to 24 hours.
When ready to serve, bake covered for 25 minutes, then remove the foil and bake an additional 10-15 minutes until the cheese is melted and bubbly.
Let the dish rest for 5 minutes before serving.