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, melt the butter and add the onions. Cook the onions until tender, about 5 minutes. Stir in the flour and seasonings, cook for an additional 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, broccoli, and cheese sauce. Divide the mixture between the prepared casserole dishes.
Sprinkle the remaining cheese over the top.
Bake the casseroles for 45 to 50 minutes or until the edges are bubbling.