Mix together chicken, taco seasoning, corn and black beans
Melt butter in saucepan over medium high heat
Whisk in flour until smooth
Slowly whisk in broth and bring to a simmer for 4-5 minutes until thickened
Stir in sour cream and green chilis until smooth and remove from heat
Place 1/4 cup sauce in bottom of round cheesecake pan with removable bottom that will fit in pressure cooker
Add 3 tortillas, cut to fit if needed
Add half of chicken filling on top
Top with 3 additional tortillas
Top with remaining filling and sauce, then with final 3 tortillas
Top with cheese
Freeze for up to 3 months until ready to cook
Place 1 cup water in bottom of pressure cooker
Add trivet and place frozen casserole in pan on top
Set to high pressure for 20 minutes and allow natural pressure release for 10
Remove casserole carefully from pot with tongs or potholders
Allow to rest 10 minutes before slicing and serving