Pour milk, eggs and seasonings into a large bowl. Whisk until mixed together.
Empty the spinach into a fine sieve and push down with your palm until it is well drained. I even squeeze it out at this point. Add it to the egg mixture along with the cheese, and mix.
Fry the bacon, onions and mushrooms until well cooked, but not very crispy.
While this is frying, place the thawed pastry shell into a glass pie plate.
When the bacon is cooked, add it to the bowl and mix it all well. Pour everything into the pie shell. I usually push all the ingredients slightly to the sides so that the very center of the pie is less deep and will cook evenly.
Place in the center of the oven for 40 minutes or until the center of the quiche is set.
Remove from oven and let stand for about 5 minutes before slicing.