Get the pasta cooking in a very large pot of salted boiling water. When it's finished, drain pasta from the pot, setting aside about 1/2 cup of pasta water. Return pasta to large pot and set aside.
In a medium saucepan, melt the butter over medium heat. Add garlic and cook for about 1-2 minutes. Add a pinch of salt and the spinach to the saucepan (Yes, the pan will be full. Just stuff it all in.) Cover pot and reduce heat to medium-low. Let the mixture cook for about 4-5 minutes or until the spinach is completely wilted.
Add the yogurt, avocado, lemon and 2 Tablespoons of reserved pasta water to the spinach mixture. Stir it a bit, then buzz it up with an immersion blender. (If you don't have one, transfer to a food processor or a blender. Blend well, and return sauce to the pot.) Stir in the cheese, and add more pasta water if it seems too thick. Add salt and pepper generously to taste.
If your pasta is still hot, simply toss the pasta and sauce together in a large serving bowl. If it's not, then combine the past and sauce in one of the pots (preferably the larger one) and cook over medium low until everything is hot. To serve sprinkle with grated cheese.