Place the mashed potatoes (while still hot/warm) into a large bowl and spread them out to release the steam. Next, add the pumpkin, egg, salt and nutmeg and mix well.
Add the flour, potato starch and grated cheese and continue to mix.
When it comes together, place on a flour dusted board or countertop and knead gently to form a smooth dough.
Put the dough onto a well-floured countertop or work area and knead gently until the dough has a uniform texture. Cut off a piece of dough and leave the rest on the counter with the bowl covering it (upside-down).
Roll the piece of dough into a rope and cut off pieces (about ½" to ¾" long) with a small, sharp knife.
Shape the pieces into gnocchi on a fork or a gnocchi board using gentle pressure, and easing up as you roll downwards. Use enough flour so that they don't stick, then place on a semolina covered tray or countertop.
(Make the butter-sage sauce if cooking immediately.)
If not cooking the potato pumpkin gnocchi immediately, you can freeze them on the tray, then place into a freezer bag when frozen, and store in the freezer for up to 2 weeks.
Butter Sage Sauce: Melt the butter (as much as you need for the amount of gnocchi you are cooking) with the sage in a sauté pan.
To cook the gnocchi: drop into the salted, boiling water and remove with a slotted spoon as soon as they float to the top or within 30 seconds, or so. Gently place the potato pumpkin gnocchi into a the pan with the butter sage sauce, and toss gently.
Serve immediately.