Begin by preparing your cold, boiled potatoes. If you’re using leftover potatoes, crush them gently with a fork or potato masher. Ensure they’re cool and slightly firm for best results.

In a large skillet, heat a little of the fat (butter, duck fat, or goose fat) over medium heat. Add the chopped bacon (or any leftover meats you're using) and fry until crispy. This should take about 5 minutes. Remove the bacon and set aside.

In the same skillet, add the remaining fat and sauté the finely chopped onion over medium heat for about 3-4 minutes until softened and translucent. If you're using cabbage, add it now and cook for another 4-5 minutes until tender. If you're using Brussels sprouts, add them to the pan and cook until softened, about 3 minutes.

Add the crushed potatoes to the pan, along with the crispy bacon, onion, and cabbage (or Brussels sprouts). Use a spatula to gently stir everything together. Season with salt and freshly ground black pepper to taste.

Press the mixture down into the pan with your spatula to form a flat, even layer. Let it cook for about 5-7 minutes without stirring so that the bottom crisps up. Once the bottom is golden and crispy, flip portions of the mixture over to allow the other side to crisp up as well. This can take another 5-7 minutes.

Once both sides are golden and crispy, serve immediately. If desired, top each portion with a fried or poached egg for an extra touch of richness.