Will have to give this a try...but not on my grilled cheese!
I had to follow up...No meat for me since its Friday so I had to give the grilled cheese / mayo a try.
For the first picture I tried to show how much butter and mayo used. Full sandwich was butter and half is mayo (still didn't trust it)
The second picture shows how it did on the griddle
The third image is the dunk test w tomato soup
Results: On the griddle they fried up about the same although I think the mayo sandwich did cook up a little quicker. On taste, there was a very slight difference in taste but I would wager that if you blindfolded me I wouldn't have known the difference. Both had the right about of "crunch" and both held up as expected after dunking in the soup. I don't think I would do it again but its nice to know I can still have a grilled cheese sandwich if I am out of butter.
I will try this on the grill at a later time.