Preheat oven to 350°F. Grease a large baking dish and set aside.
In a large skillet over medium heat, add butter.
Once melted, add onion and broccoli and cook for a few minutes until softened. Add flour, garlic, and black pepper. Sauté for 2 minutes.
Add milk and whisk constantly until combined.
Remove from heat, and add mustard, paprika, cream cheese. Mix together.
Add 1 ½ cups of cheddar cheese. Stir constantly until the cheese is melted and the mixture has thickened. Make sure the cheese mixture is quite thick.
Add in room temperature rice. Mix until combined.
Pour the mixture into the greased casserole dish. Sprinkle the remaining cheese over the top.
Bake for 30-35 minutes or until the cheese has completely melted and the casserole is bubbly. Serve and enjoy!