Preheat the oven to 375ºF and spray a 9×13-inch casserole dish with non-stick cooking spray
Heat a large dutch oven over medium/high heat and add olive oil, onion, garlic, celery, carrots, mushrooms, garlic powder, and onion powder to the dutch oven and cook for 5 minutes
Sprinkle flour over the vegetables and use a wooden spoon to mix the flour with all of the vegetables until the flour absorbs all of the excess liquid in the pot
Turn heat to medium and slowly add the milk to the pot, stirring constantly until milk thickens and remove dutch oven from heat
Add in cooked rice, frozen peas, and chicken to the greased casserole dish and mix
Next add the contents of the dutch oven to the casserole dish and mix again
Add the broth, salt and pepper to the casserole dish and mix until everything is combined
Cover the casserole dish with tin foil and place in the oven at 375ºF for 30 minutes
At 30 minutes, remove the tin foil and bake for an additional 30 minutes (I like to mix around the contents and stir every 10 minutes or so)
Remove casserole dish from the oven and let sit for at least 10 minutes to fully thicken (it’s okay if there is still liquid – it keeps the dish moist especially for leftovers)
Serve while warm!
Notes
*Store in fridge for 5 days or freezer in freezer-friendly container for 2 months