Over medium heat, add milk and half an onion to the pot. For added flavor stick 2-3 cloves in the onion. Also season with salt an white pepper to taste. Keep in mind that cheese will be added later.
In another pan, melt butter and whisk the flour.
Once the milk reaches a simmer, remove the onion and cloves in the milk and slowly whisk in the roux (butter and flour mixture). Then add the grated cheese.
Add the thawed spinach to béchamel. Be sure that you squeeze out all of the water from the frozen spinach before adding it to the béchamel sauce.
Add the creamy spinach mixture to the pie crust and bake. This is a pre made frozen pie crust that I bought at my local grocery store