Preheat oven to 350.
Thaw and drain spinach (you'll want to drain the spinach really well- squeeze out any extra liquid by draining spinach in a colander and pressing out as much liquid as possible.)
Mix together spinach, cream cheese, mushroom and 1/2 of the French's onions.
Pour into greased casserole dish.
Add remaining French's onions.
Bake for 20 minutes, or until heated through (make sure not to let onions brown too much.)
Enjoy!