Preheat oven to 350F.

Cook spinach according to pkg directions, and squeeze dry.

Mix ricotta, eggs, onion salt, and Parmesan cheese (if desired). Add squeezed chopped spinach and mix well.

Line pie plate with Muenster slices, add spinach mixture, cover top with more Muenster cheese slices.

Bake 45 mins. until golden brown and firm.

Let cool slightly before slicing into 8 peices.