Preheat the oven to 400 degrees. Coat a 9 x 13 glass baking dish with cooking spray.
Prepare the broccoli by filling a bowl with ice water; set aside. Boil a saucepan of water over high heat. Add the broccoli florets and cook for 30 seconds.
Using a slotted spoon, remove the broccoli florets and immediately plunge them into the ice water to stop the cooking process and preserve the bright green color.
Remove the cooled broccoli florets to a plate lined with paper towels to drain well.
Prepare the chicken by heating 1 tablespoon of butter and 1 teaspoon of olive oil in a large skillet over medium-high heat.
Add the diced chicken breasts and season with sea salt and freshly cracked pepper, to taste.
Cook, stirring occasionally, for 4-5 minutes, or until just cooked through. Transfer the chicken and juices to a bowl; set aside.
Prepare the mushrooms and onions by heating two tablespoons of butter in the same large skillet over medium-high heat.
Add the mushrooms and onion then cook, stirring occasionally, for 5-6 minutes, or until the onions are softened and the mushrooms are golden.
Add the minced garlic and cook, stirring constantly, for 45 seconds. Season with sea salt and freshly cracked pepper, to taste.Transfer to the same bowl as the chicken; set aside.
Make the sauce by heating the final three tablespoons of butter in the same large skillet over medium-high heat.
Add the flour once the butter has melted and whisk together constantly for 1-2 minutes, until starting to turn golden and smell nutty.
Slowly add the milk in a steady stream while continually whisking. Allow the sauce to come to a simmer and thicken a bit for a few minutes.
Whisk in 2 cups of the sharp cheddar, a handful at a time, making sure the cheese completely melts before adding another handful. Remove from heat.
Taste and season with sea salt and freshly cracked pepper, to taste.
Finish the casserole by adding the chicken, mushrooms, onion, broccoli florets, and leftover rice to the cheese sauce and stir until combined.
Pour into the prepared baking dish and spread out evenly.
Next sprinkle the remaining 1 cup of cheese evenly on top of the casserole.
Make the topping by combining the crushed Ritz crackers with the melted butter; mix until well combined.
Sprinkle evenly on top of the casserole.
Place into the oven and bake for 15-20 minutes, or until browned and bubbly.
Allow to sit for 15 minutes before serving. Enjoy.