Start heating a large pot of salted water.
Blend the ricotta and the egg together in a large bowl. Mix in the spinach, cheese, bread crumbs, flour, salt and pepper, and knead lightly.
Test the consistency of the dough by scooping up a heaping tablespoon, forming it into a ball, and flouring it. Drop it into the boiling water; if it does not hold its shape and rise to the surface of the water within a minute, add more bread crumbs to your dough.
When you have the right consistency, shape all of the dough into balls the size of golf balls, roll them lightly in flour, and lay them out on baking sheets covered in parchment paper.
Drop the gnudi gently one by one into the boiling water and cook for about 2 or 3 minutes, until they rise to the top, and come to a rolling boil. To test for doneness, scoop out a ball and press it with your fingers: the dumpling dough when cooked should bounce back, leaving no indentation. Gnudi would go nicely with a butter and sage sauce. Top with freshly grated cheese before serving.