Bring a large pot of water to a boil. Submerge the spinach and allow to cook for a minute. Retrieve with a slotted spoon or strainer. Allow to cool slightly and chop coarsely, allowing excess water to drip off. Set aside. Retain the water in the pot to cook the pasta.
In a large skillet, over medium heat, sauté the onion in olive oil until golden. Add the spinach, salt and pepper to taste. Mix together and cover. Remove from heat.
Bring the pot of water to a boil, adding salt for cooking the pasta. Cook the pasta until al dente, drain, retaining some of the cooking water. Add to the pasta to the skillet with the spinach, now over medium heat, along with some of the retained cooking water if it seems dry.
Serve immediately with a sprinkle of Pecorino cheese.