how to create gratin with _thin_ cheese skin
I would think all of the above will give tasty results, the point about the use of a professional broiler or salamander is possibly the real clue for cheese to crisp it needs heat, the rest of the dish probably does not.

There are a few tricks to try, 

1 use any of the above mixes but get yourself a blow lamp from the DIY store or garage, not a silly little overly priced armature cook shop one, and use it to flame the tops just before serving(warn guests of the temperature before they burn their mouths) Effectively same as a good crème brulee. 

2 make parmigiana disks grate parmigiana very finely on to a silicon tray or baking sheet oven roast until crispy or use grill if you have a hot broiler. let them cool on a rack place them on top at the end of the cooking - Combine with the blow lamp to colour and heat as a professional touch.

3 the lazy way add a dash of butter to the tops during the baking to colour the cheese mixture.