Creamy Chicken and Rice Casserole {without soup}

Melt butter in a large pot over medium heat and cook onions until translucent. Add garlic, seasoned salt, pepper. Add flour and mix well. Cook until starting to color.Stir milk and stock into flour mixture. Turn heat to medium high and continue to whisk as mixture comes to a simmer. Taste and adjust seasoning. Add rice and lower heat to medium low. Cook until the rice absorbs most of the liquid and is tender, stirring occasionally about 20 minutes.Preheat oven to 350 degrees. Add  chicken to the rice and sauce mixture. Pour mixture into a 9 x 13 inch casserole, or two 8×8 inch casseroles.Combine breadcrumbs and cheeses and sprinkle evenly over top of the casserole. Place in hot oven and cook for 20 to 25 minutes or until topping is browned and casserole is bubbling.