Blitz the tomatillos and sage leaves in a blender until smooth.
Set a large frying pan with the oil over a medium heat. Cook the onion for 5 minutes until softened, then stir in the garlic and chilli, if using, and cook for 30 seconds more.
Stir in the spinach and blended tomatillos. Cook for 3-5 minutes, stirring occasionally, until the spinach has wilted and the sauce has thickened slightly. Season.
Use a spoon to make 5 shallow wells in the sauce. Carefully crack an egg into each one, splash with a little water and sprinkle with the parmesan. Cover and cook for 8-10 minutes, or until the eggs are done to your liking.