Preheat the oven to 425 degrees, rinse potato with water, pat dry, and use a fork to poke holes all around the potato (for even cooking). Then place potato on a parchment lined sheet pan. Roast for 30-40 minutes until fork easily slides into the center.
Take potato out of oven and carefully remove skins with a fork and tongs.
Add the peeled potato to a glass mixing bowl and mash with a fork or potato masher. Let cool for 5 minutes
Add sourdough starter, salt, pepper, egg, flour, and shredded/grated parmesan cheese to the bowl. Mix together with the fork or your hands.
Turn dough out on a lightly floured workspace and gently knead for about 1 minute. Add a little flour if dough is sticking to hands and workspace.
Cut dough in half and roll each half into a 12-inch rope about 1 inch wide. Cut each rope into 1/2-inch pieces.
Add gnocchi to boiling water and cook for 5-7 minutes on medium heat then add to a pasta sauce. Or add to a soup that's simmering and cook for 5-7 minutes until the gnocchi float to the top.