Bring 750ml of water to a boil and add a pinch of salt.
Stir in the 200g of polenta and simmer for 4-5 minutes until smooth and thick.
Mix in the chopped olives and season with salt and pepper.
Spread the polenta evenly onto a lined baking tray and leave to cool and set.
Cut the set polenta into 10cm squares and brush both sides with olive oil.
Griddle the squares in a hot pan for 2-3 minutes per side until golden marks appear.
Sauté the garlic and 450g of mushrooms in oil and butter for 3-4 minutes until browned.
Add the lemon juice, zest, and thyme to the mushrooms and stir well.
Serve polenta squares topped with mushrooms, pan juices, and parmesan shavings.