Mix the potato slices with the cheese, the pith, the marjoram and some salt.
Lightly beat the eggs to incorporate some air into them, which will make for a fluffier frittata. Add the potato mix.
Pour the mixture into the greased skillet, give the skillet a brisk shake to distribute them evenly (this omelette should be 3 cm /1" high), and let the frittata cook, without stirring, till the top begins to firm up. Cover the skillet with a lid that doesn't have a lip and flip it upside-down while holding the lid pressed to it (you may want to do this over a sink the first time).
The frittata will come to rest on the lid; turn the skillet right-side up, slide the frittata back into it, and cook it for a couple of minutes more to set it through. It shouldn't be runny, nor should it be bone dry.
As a variation in technique, if your skillet has heatproof handles you can slip it into the oven for a few minutes once the top begins to set. In either case, slice the frittata into wedges as you might with a pizza before you serve it.
Serve warm or room temperature.