I'd have gone with cheese, ketchup and mustard myself - I don't think mayonnaise has any place on a burger, OH insists on putting mayo on the bottom bun though for some reason. Also there shouldn't be so much in the way of condiments that it makes the bun soggy or drips everywhere (keeping it as something you can pick up and eat on the go), a small bit of each is sufficient.
So my version of that sort of perfect, simple, non-gentrified burger would be (in order of the items, and the bun is standard plain or sesame seed, not brioche, if I want dessert I will ask for it):
Top of the bun
Ketchup and Mustard
Onions
Cheese slice
Burger
Bottom of the bun