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 the 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 for about 20 minutes.
Preheat oven to 350 degrees.
Add chicken to the rice and sauce mixture and mix well.
Pour mixture into a 9 x 13-inch casserole, or two 8x8 inch casseroles.
Combine cracker crumbs and cheeses and sprinkle evenly over top of the casserole.
Place in a hot oven and cook for 20 to 25 minutes or until topping is browned and casserole is bubbling.