Preheat oven to 350ºF and grease a muffin pan.
If using fresh spinach, heat greens in medium saucepan over medium heat, stirring occasionally until wilted. Set aside to cool. If using frozen spinach, cook over medium just enough to defrost.
Meanwhile, carefully line each muffin cup with a prosciutto square, pressing the prosciutto all the way down into each cup.
Combine remaining ingredients in a blender and mix until relatively smooth.
Using your hands, squeeze remaining water out of cooled spinach and roughly chop. If using fresh spinach, it should have reduced to about ¼ cup. Throw the spinach into the mixture in the blender and stir to incorporate (don’t blend).
Pour the mixture into the prepared prosciutto cups. Each cup should be just barely filled.
Place in middle rack of oven. To prevent the eggs from drying out and to lend them a nice fluffy texture, pour a couple of inches of water into a 13x9-inch pan and place it on the rack under the eggs. Bake for 20 minutes, or until the eggs fluff up, tops are just beginning to brown and visible prosciutto is nice and crispy.
Let eggs cool for about 10 minutes before carefully lifting out of pan with a fork or small spatula.