In a large pot, cover potato and plantain with cold water. Bring to a boil and cook until plantain is tender, then remove. Continue cooking the potato until tender, about 20 minutes total.
Once potato and plantain are cooked and cooled, remove skins. Pass through a ricer. Make sure they’re cool enough to touch.
In a bowl, mix the riced potato/plantain with the rest of the ingredients (1/2 cup flour at a time) until well combined and a dough is formed, about 3 minutes. if dough is too sticky, add more flour.
Cut dough ball into 8 even pieces. On a floured surface, use your hands to roll one dough ball into a long cylinder/snake shape about 1/4 inch in diameter. Use a pastry scraper or knife to cut 1/2-inch pillows. Holding a fork at a 45-degree angle, roll the gnocchi pillows off the back of the fork to create the traditional gnocchi divots. Cover gnocchi with a dish towel while you finish making the rest.
Bring a large pot of salted water to a boil. Add gnocchi and cook until they float to the top. Cook for 1 minute longer, strain and serve plain or with desired sauce.