Very tasty. I too used forzen spinach which worked out well. The only thing I would change is the amount of butter. I think (half the amount) 2 oz. would have been fine. The choice of cheese was good. My husband, who presumbaly, doesn't like spinach thought these were pretty tasty.
These were good - my husband kept snatching squares out of the refridgerator all day! I thought them just a bit bland by themselves (were great with spaghetti sauce to dip them in) but I think some garlic and maybe a bit of cayenne powder will add a lot of flavor. Jack cheese was a great choice. I used frozen spinach as that's what I had; the finshed product is a very pretty dish (some red, maybe diced pimentoes, would look lovely, too.)