Oooo. I feel a little like a Great British Bakeoff contestant in the ‘technical’ round. A presenter has just wheeled out the fancy French title of a dish, and I’m about to have to try to cook something I’ve never even heard of before. 
As a fan of cooked cheese, and suffering my ancestral weakness for anything deep-fried (https://twitter.com/qikipedia/status/121233925250088961), I’ll admit to being rather intrigued! Yup, that looks to me like very similar sort of cheese/stuff ration to a ‘traditional’ rarebit. Not that I’ve tried making it with gruyère, but I’m assuming it’d give similar results.
So I don’t know if rarebit would quite live up to that sybaritic comparison. OTOH, it’s a whole lot less work to make, without having to make the batter and to deep-fry it.
I can see the attractions of the Mornay sauce approach for the restaurant – have a vat of that on hand, make the toast, pour, done. Or maybe it’s seen as ‘fancier’ than the doorstep of bread and thick cheese goo with ad hoc blisters from under the grill? Not at all what I’d have been hoping for those, I confess.
The variations in this article all seem recognisable to me: https://www.theguardian.com/lifeandstyle/wordofmouth/2011/oct/27/how-to-cook-perfect-welsh-rarebit (It’s one of the Guardian’s frequent tricks of getting a long article out of a very short and pretty simple recipe, but at least under the guise of ‘survey of the field’, rather than those annoying food-blog posts that just waffle and spam on to no purpose.) I commend it in the spirit of culinary experimentation!
I do admit to a slight reservation about the eggs, as I don’t use those myself, and adding a protein-based sauce element seem to be unnecessarily adding a whole extra failure mode to an otherwise very straightforward recipe. OTOH the ‘buck rarebit’ with an added poached egg – now we’re talking!
