Once you get the hang of shaping the potato croquettes, papas rellenas are very easy to make.
1 to 3 Season the riced potatoes with salt, add the oil, and use the palm of your hands to knead the potatoes for 2 or 3 minutes, until they become almost silky to the touch.
4 Form about ¼ cup of the potato base into a 4- to 5-inch oblong disc in the palm of one hand.
5 Place about 2 generous tablespoons of the filling in the center.
6 Add an olive and quartered hard-boiled egg, if using.
7 Gently press the potato base over the filing to close, like you are closing a clamshell.
8 Pass the rellena back and forth between your hands to shape it into an oblong round the shape of a russet potato or football.
9 Add a little more potato “dough” to patch up any cracks on the outside, if necessary.