Thaw the frozen spinach completely.

Squeeze all excess water from the thawed spinach.

In a mixing bowl, combine the squeezed spinach, vegetable soup mix, sour cream, mayonnaise, and finely chopped onion.

Mix all ingredients thoroughly until well combined.

Cover the bowl and refrigerate the mixture to cool and allow the flavors to blend, about 45 minutes.

Cut the Hawaiian bread loaf crossways three times, creating four layers.

Spread a generous layer of the spinach mixture between each bread layer.

Stack the bread layers on top of each other, forming a sandwich cake.

Wrap the assembled sandwich tightly with plastic wrap or foil.

Refrigerate the sandwich for at least 30 minutes to allow the filling to set and the flavors to meld.

Slice and serve chilled.