Preheat oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with butter or cooking spray.

In a large pot or Dutch oven over medium-high heat, brown the ground beef with salt, pepper, and Italian seasoning for 7-8 minutes, breaking it into small crumbles. Drain excess grease, leaving about 1 tablespoon of fat. Transfer beef to a plate and set aside.

Reduce heat to medium. Add 2 tablespoons butter to the same pot. Once melted, add diced onions and cook for 5 minutes until soft and translucent. Add minced garlic and cook for 1 minute until fragrant, stirring constantly.

Pour in beef broth, scraping the bottom of the pot to release browned bits. Add remaining 1 tablespoon butter and uncooked rice. Stir and bring to a boil over high heat. Once boiling, reduce heat to low, cover tightly with lid, and simmer for exactly 10 minutes without lifting the lid.

Quickly lift lid and scatter sliced mushrooms evenly over the rice surface. Do not stir. Replace lid immediately and continue simmering on low for 5-10 minutes until liquid is absorbed and rice is tender. Turn off heat and let sit covered for 10 minutes.

Remove lid and fluff rice with a fork, incorporating mushrooms. Add cooked beef, cream of mushroom soup, milk, sour cream, and 1 cup shredded cheddar cheese. Stir until well combined and creamy. Taste and adjust seasoning with salt, pepper, and cumin as desired.

Transfer mixture to prepared baking dish and spread evenly. Sprinkle remaining 1 cup cheddar cheese across the top in an even layer, covering to the edges.

Bake for 25-30 minutes until cheese is melted, bubbly, and golden brown in spots. Let rest for 5 minutes before serving.