In a large bowl, combine mashed potatoes, egg yolk, ½ cup ricotta, salt and ½ cup flour; stir to combine before adding the remaining flour and mix again.
On a very well-floured work surface, dump on the potato mixture. Add more flour and knead until no longer sticky (add as much flour as needed).
Shape into a ball and divide by 4. Roll a ¼ at a time into a long tube of 1-inch, doing a back and forth motion by putting a little pressure on the middle and working your way out to the ends.
Flour a bench scraper and cut the dough into 1-inch long pieces.
With the back of a well-floured fork, press down on each gnocchi.
Place gnocchi on a large baking sheet lined with a silicone mat or wax paper and transfer to the fridge for 40 minutes.
Bring a large pot of salted water to a boil.
Working in batches, drop gnocchi in and as soon as they rise, remove them using a spider strainer.
Transfer them to the sauce. Stir until gnocchi are well coated.
Spoon onto a plate and sprinkle Parmesan cheese on top.