Pan fry the bacon in a little oil for about ten minutes or until starting to brown/crisp. Remove the bacon from the pan with a slotted spoon, leaving any pan juices behind.
Meanwhile, slice your potatoes thinly (about 3 to 4mm thick). I use a food processor to get even slices, but this isn’t essential.
Now slowly cook the onions in the butter in same pan you cooked the bacon in until they start to go golden and sticky (about 15 to 20 minutes). Stir from time to time (but not too much). Just before the end of the cooking time, add a splash of balsamic vinegar and let cook for a minute or two more.
Preheat the oven to 190C.
While waiting for the oven to heat up, layer up your potato bake like a lasagna. Spray your baking dish with oil (a medium sized dish - I used the equivalent of a 12 x 9 inch dish). Cover the bottom of the dish with about ¼ of the potato slices. Sprinkle with 1/3 of the onions, 1/3 of the bacon, and ¼ of the parmesan. Sprinkle over a large pinch of herbs, and plenty of black pepper.
Continue layering until you have a fourth and final layer of potatoes on the top of your casserole. Press the potatoes down with your fingers, then sprinkle with the final ¼ of the parmesan.
Place the dish on a baking sheet, then carefully pour over the broth/stock until it barely covers the potatoes.
Dot the top of the potato bake all over with butter, then bake for 1 hour to 1 hour and a quarter or until your potato bake is nicely browned on top and bubbling, most of the broth/stock has been absorbed, and the potatoes are tender.
Let stand for 20 minutes to let more of the broth/stock be absorbed. Sprinkle with a little chopped fresh parsley if you like before serving.