Melt the butter in a saucepan over medium heat and saute the onions until translucent.
Slowly whisk in the chicken broth and milk. Stir over medium heat for 4-5 minutes until it starts to bubble and thickens. Remove from heat.
Stir in the cream cheese, mixed vegetables, and chicken.
Pour chicken mixture into a 9×13 casserole dish.
Add the frozen tater tots in a single layer over the chicken pot pie mixture. Bake for 40 minutes.
Remove the casserole from the oven, top it with freshly grated cheese, and return to the oven for an additional 3-5 minutes.
Serve this tater tot chicken pot pie casserole immediately.