(Polenta can be prepared the night before or early morning of the day you want to serve this dish so it has plenty of time to cool and set)
Line a rimmed baking tray with baking paper. Set aside.Add water to a large pot and whisk in the polenta. Bring to a boil, stirring occasionally. When the water comes to a boil, stir continuously until the polenta becomes thick and starts to bubble. Reduce to a simmer and cook for 10 minutes.
Switch off the heat and stir in the butter and Parmesan. Season to taste. Scrape polenta into the prepared baking tray and smooth. Cover with cling film and place in the fridge until completely firm.
For the mushrooms and spinach:
Sautée spinach in butter and garlic until just wilted. Season to taste and set aside. Sautée the mushrooms and shallot on high heat in the butter and olive oil. When golden brown add the vinegar and stir. Switch off the heat and add the thyme and chilli flakes. Mix and season to taste.
Preheat oven on the grill setting.Remove polenta from the fridge and cut into 8 neat squares. (Snack on the offcuts.)
Pan sear the polenta in a hot pan with a little butter until golden brown on both sides. Flip polenta only once as it will become fragile. Place polenta squares onto a lined baking tray.
Top with cooked spinach, mushrooms and mozzarella cheese. Place under the grill for a few minutes until the mozzarella is bubbling and melted. Serve 2 squares per person with a green salad.