Place the whole unpeeled potato in a pot and cover with cold water by a couple of inches. Bring to a rolling boil, lower the heat to a gentle boil and cook until the potato is easily pierced with a skewer, it will take 40-50 minutes.
For Fresh Spinach: Heat the butter in a small fry pan, add the spinach and sauté until wilted, about 2-3 minutes, let it cool and transfer the spinach to a bed of paper towels. Roll the paper towel up around the spinach to absorb the excess moisture, chop finely and place in a mixing bowl. If Using Frozen Spinach: Defrost the spinach (in the microwave) and place on a paper towel to squeeze excess water out. Chop finely and place in a mixing bowl.
Add the ricotta, egg, and salt to the mixing bowl with the spinach, mix with a fork until well combined.
When the potato is cool enough to handle, I use gloves because it is better to do this when the potato is still warm, peel it and “rice” the potato or mash it. I do this onto a piece of parchment, so that I can spread the potato out so that it cools.
Add the potato to the spinach mixture, gently mix together, add the flour (no more than ¾ cup), mix gently, try to mix well without over mixing. Use a for to mix and mash.
Dust a rimmed baking sheet generously with flour. Using 2 large soup spoons, shape heaping tablespoonfuls of dough into oval shapes; place on baking sheet and dust with more flour (you should have 30).
To Cook: Bring a large pot of water to boil, add salt (about 1 Tbs.) add g'nudi (about 10 at a time). Stir occasionally. Cook for about 5-6 minutes, they will float to the top, drain using a slotted spoon and transfer to bowls.
Spoon Pomodoro or Arrabbiata Sauce, spooned over the g’nudi and serve with grated parmesan cheese.