Preheat the oven to 350 degrees. Prepare two 9x13 casserole dishes with nonstick cooking spray.
In a large (12 inch) skillet over medium-low heat, brown the ground beef with the onions, crumbling the beef as it cooks. Drain the beef well.
As the beef drains, add the butter to the skillet and melt. Stir in the flour and seasonings, cooking for about 2 minutes.
Gradually add in the milk, whisking until thick and bubbly.
Remove the skillet from heat. Stir in the cream cheese and half of the cheddar cheese until melted.
In a 30 quart bowl, combine the rice, beef, and cheese sauce. Divide the mixture between the prepared casserole dishes. Top with the remaining cheese.
Bake the casseroles for 45 to 50 minutes or until the edges are bubbling.