Add a couple of lettuce leaves and the top of the bun, with a second dash of mustard if you like.
Should be eaten right away.
Remarks
There are lots of ways to vary this basic idea, still in keeping with the Mexican spirit: bacon instead of ham, a different cheese, a dash of Tabasco or lime juice on the bean patty, add a little avocado, etc. Just let your imagination and preferences guide you.
If you are fortunate enough to be in Brest, France, trot along to the remarkable "Pen ar bread" bakery in Guipavas, run by Sébastien and Audrey. I recommend it wholeheartedly (Brittany's best bakery in 2019). Here you'll find a whole range of delicious burger buns, like the ones shown in the photos. The owner is a really nice guy and a fount of knowledge about the science of baking, and happy to talk about it.