In a pot, add the olive oil, the minced garlic and the tomato sauce. Bring to a simmer then add the basil leaves and lower the heat. Simmer for about 20 minutes. Season with salt and pepper to taste.
Make the Gnudi
In a pan, drizzle the olive oil and add the halved garlic clove and the spinach. Season with salt, pepper and ground nutmeg and sautè for about 5 minutes.
Drain the excess liquids as much as possible, by squeezing the spinach, then set them aside in a bowl to cool down.
Meanwhile, bring a large pot of salted water to boil.
Add the cooled spinach to the ricotta cheese, add the Parmigiano cheese and season again if needed. Then add the egg yolk and half of the flour.
Mix all the ingredients and add more flour until you get a thicker consistency.
With a tablespoon, make the balls and roll them in flour. Set them on a plate or floured surface to rest. If they seem too soft, you may want to test one in boiling water. If it melts and doesn't keep the shape, you will need to add more flour to the dough.
When they're all ready, toss them gently in the boiling water one by one.
Boil for 2-3 minutes, until they come up to the surface. Remove them with a skimmer and add them to the tomato sauce or directly in the serving plates.
Notes
You can substitute the frozen spinach with 2 lb (1 kg) of fresh spinach.