Preheat the oven to 250°F. Set a wire cooling rack on top of a baking sheet and set aside.
Bring a large pot of water to a boil and add the potatoes. Cook until a knife can be inserted into the center of each potato without much resistance. Drain and set aside to cool. Once potatoes are cool enough to handle, peel, and place in a bowl. Cover with plastic wrap and chill in the fridge for at least a few hours or overnight.
When you are ready to make your rösti, start by grating the potatoes on the side of a box grater with the largest holes. Season the grated potatoes generously with salt. Heat a small (8-inch) skillet over medium heat. Ideally you want a low-rimmed skillet that will allow the potato pancake to slide right off. Add about one teaspoon of goose fat and one teaspoon of butter to the pan. Once melted and sizzling, add 1 ½ cups of the grated and seasoned potatoes to the skillet. Use a spatula to spread the grated potato so that it evenly covers the entire surface area of the pan. Make sure to press down a bit on the potato while you do this (it is the starch in the potatoes that holds the rösti together so the potato cake needs to be packed together). Cover the skillet and cook on medium heat until the edges of the rösti are golden brown, about 7 minutes. If you find your rösti is browning too fast, lower the heat slightly. (Be careful not to lift your rösti when checking to see if the underside is browned, as it tends to fall apart if you handle it too much).
Once your rösti is golden on the edges, remove the lid and place a plate on top of your skillet. Invert the skillet so the rösti lands cooked side up on the plate. Add another teaspoon or so of goose fat and butter to the hot skillet. Slide the rösti, uncooked side down into the skillet and cook, uncovered, for another 4 minutes.
Once finished cooking, slide the rösti onto the wire rack lined baking sheet and place in the oven to keep warm. Repeat the process with the remaining rösti, adding each to the oven to keep warm before making the next.
In large pan over medium heat, melt two tablespoons of butter. Add the spinach and cook for about 5 minutes. Stir half way through. Once wilted, season the spinach with salt and pepper and keep warm.
To serve, transfer the rösti to individual serving plates. Top each rösti with some spinach, smoked salmon and a tablespoon of crème fraiche. Enjoy immediately.
The rösti is a versatile base for all different kinds of toppings. A fried egg is a delicious and simple alternative.