Preheat your oven to 350°F and grease a 9×13-inch baking dish with non-stick cooking spray or butter.

Melt the butter in a large skillet over medium-high heat. Add the diced onions and sauté for 3 to 4 minutes, stirring frequently, until they become tender and translucent.

Add the ground beef and garlic to the skillet with the onions. Cook the meat for 6 to 8 minutes, breaking it apart into small, uniform crumbles using a wooden spoon, until it is completely browned. Drain away all excess grease.

Reduce the skillet heat to medium. Season the beef with salt, black pepper, and dried oregano to taste.

The Starch Infusion Simmer: Pour the tomato sauce, chicken broth, the cooked rice, and exactly 1 cup of the shredded cheddar cheese directly into the hot beef skillet. Stir continuously and let the mixture simmer gently for 3 minutes. Pro Tip: Simmering already-cooked rice directly inside the acidic tomato broth allows the grains to drink up the savory liquid baseline, plumping them further and binding the loose components into a cohesive filling before it ever enters the oven.

Pour the hot, bubbling meat and rice mixture into the prepared 9×13-inch baking dish, smoothing it into an even layer. Pop it into the oven uncovered for 15 minutes.

While the base bakes, vigorously stir together the room-temperature sour cream and the remaining 1 cup of shredded cheddar cheese in a small bowl until it forms a thick paste.

The Stabilized Dairy Blanket: At the 15-minute mark, pull the baking dish out. Carefully spread the sour cream and cheese paste evenly over the top surface. Return to the oven for an additional 8 to 10 minutes until the top is entirely melted and bubbling at the perimeter. Crucial: Spreading sour cream as a dedicated top blanket layer later in the baking cycle insulates the dairy proteins, preventing them from breaking down, curdling, or sweating into watery pockets. Let it rest for 5 minutes to set before serving.