Preheat the oven to 400f.
Wipe the mushrooms clean and remove the stem centres.
Place onto a baking sheet.
Place the spinach into a wide bottomed pan with a splash of water and cook over a medium high heat until the spinach has wilted.
Allow to cool a little.
Place the ricotta cheese, egg, mozzarella cheese and parmesan cheese into a bowl, season with salt and pepper.
Squeeze any excess water out of the spinach then chop it into smaller pieces to make it easier to incorporate into the cheese mixture.
Add in with the cheeses and stir to combine.
Spoon generous amounts of the mixture on top of each mushroom. Press down slightly to create a flat surface.
Spoon over some marinara sauce then top with some extra mozzarella.
Place in the oven for 20-25 minutes until the mushrooms are tender and the cheeses melted and bubbling.
Serve with a fresh green salad.