In a pan add oil to caramelize the onions celery bell peppers

After a min of cooking add mushrooms and vinegar and rotel tomatoes

Then make rue by adding flour after its golden add chicken stock then add milk

Cook on low until it is viscous

In a casserole dish put some of the sauce on the bottom of the dish then a layer of tortillas. on the layer of tortillas put a layer of chicken and then top that with cheese and more sauce then repeat this step until a 1/4 inch from top and just top with more cheese

Bake in oven at 350 until cheese is bubbly or how ever you like