Frika will invariably involve some sort of hard mountain cheese - in the Soca Valley, it will usually be Tolmin cheese, across the border in Friuli it will be Montasio, as far as I know.
The basic recipe is to sauté thin potato slices on lard (oil if you're vegetarian), preferably in a very hot cast-iron skillet until they get a consistency that is somewhat between chips and fries and then add cheese. You want to avoid mashing the potatoes and the skillet should be hot enough to for the frika to develop a nice golden crust.
However, there are many variations. Although I'm not aware of it being a common ingredient in that region, I've come across recipes that include zaseka, which is a sort of terrine of minced bacon and spices (not applicable in your case, hopefully). According to Janez Bogataj, an ethnographer and probably the expert on all things Slovene, there are many different recipes in the area around Kobarid, varying from village to village and from family to family. In Volarje, potatoes are substituted with eggs; in Poljubinj, eggs are added with the potatoes; some add onions as you've mentioned and in Kobarid proper they would add strong, salted skuta (quark). There are also some recipes involving local greens and herbs, although others would consider anything green in a frika heresy.
You could try out one of these variations but if you want the exact recipe, you can always email the family. I'm sure they will be happy to help you out. If they provide you with a recipe in Slovenian, feel free to PM me for the translation. Heck, if it was as good as you say I just might want to keep a copy for myself. ;)
