"en" would mean in. "sur" means on. You could say "On my sandwich" or "in between my sandwich(es)". But I'd say that "in my sandwich" sounds a little odd, like whatever you put on is merged with the bread of your sandwich.
Sur = on
En = in
In English, you spread butter ON the bread. But fillings like meat or cheese you put in it, as in between the slices of bread.
That said, if someone were making you a sandwich they might ask you if want meat ON it. English...