Preheat oven to 400.
Prepare stuffing according to box directions. Set aside.
In a medium skillet, heat 1 tablespoon butter.
Add in chicken and cook until no longer pink.
Place chicken evenly on bottom of a 9 x 13 baking pan.
In same skillet, add remaining 2 tablespoon butter.
Saute onion and mixed veggies for about 5 minutes over medium heat.
Add veggies/onion over chicken in pan.
In a medium pan, add the cream of chicken soup, milk, and flour.
Simmer over medium heat, stirring to avoid any lumps.
Once boiling, remove from heat and pour evenly over the chicken/vegetables.
Sprinkle cooked stuffing evenly on top.
Place pan in oven and bake for 25 minutes.
Remove from oven and serve!