Season the pork schnitzel with salt and black pepper. Heat fat in a skillet over medium-high heat. Fry the schnitzel for 3-4 minutes per side until browned. Transfer to a baking dish.
Drain any excess liquid from the mushrooms. Spread them over the schnitzel. Pour 250 ml of hollandaise sauce over the mushrooms.
Peel the potatoes, cut into slices, and spread over the mushrooms. Season each layer with salt and black pepper. Pour the remaining 250 ml hollandaise sauce over the potatoes. Top with grated Gouda cheese.
Cover the baking dish. Bake at 200°C (392°F) for 45 minutes. Remove the lid and bake for a further 15 minutes until the cheese is golden brown.