Defrost the spinach according to the package directions.
Place the thawed spinach in a tea towel with a flat weave, and squeeze it until as much of the moisture as possible has been removed.
Discard the liquid, and set the spinach aside. If your frozen spinach was whole leaf or you wilted fresh spinach instead, roughly chop the drained spinach first.
In a large mixing bowl, place the mayonnaise, sour cream and Worcestershire sauce, the onion soup mix, and black pepper. Whisk to combine well.
Add the Parmigiano-Reggiano, optional celery, and prepared spinach, and mix to combine, breaking up any clumps in the spinach.
Taste the dip and add salt to adjust for taste.
Cover the bowl and place it in the refrigerator to chill for at least an hour (to allow the dried onions to soften) and up to 3 days.
Serve in a gf bread bowl or serving bowl, with chips or celery for dipping.
Notes
Gluten free onion soup mixIf you're using my homemade version of gluten free onion soup mix, I recommend omitting the beef bouillon granules in the recipe to make this dip. I prefer the taste of the other ingredients to shine through here.Nutrition informationNutritional information is per serving assuming the whole is divided into 8 equal parts. It does not include chips or vegetables used for serving, or a bread bowl.