It's a Croatian restaurant so they probably have no idea what "kasspatzle" is - just make a special request for their spatzle with cheese and mushroom sauce/gravy on top. I'm sure they'll oblige...
Went to the New Dubrovnik last night (i.e. a Sunday) for dinner and asked for kässpätzle with my meal. Was told that the spätzle is all prepared beforehand. Perhaps this means that you have to call ahead, or go on an off-night where the spätzle is made-to-order?
Croatian restaurant New Dubrovnik (vendor 658) has spatzle as a choice of side. It's topped with grated brick cheese and a mushroom-wine sauce (close to a gravy?)
Not exactly what you're looking for but it's the first place that came to mind...
monty
gold