Place the stock in a pan with a little salt and bring to the boil. Gradually add the polenta, stirring.
Cover and allow to simmer for 15 minutes over a low heat. Stir frequently to prevent burning. Add 1 tbsp oil to the polenta. Season with nutmeg and pepper.
Grease a 32cm pizza tray, spread the out polenta mixture to about 1-2 cm thick and allow to cool.
Clean the mushrooms and halve if necessary.
Peel and finely chop the onion and garlic.
Melt the butter in a pan and saute the onions and garlic until softened. Add the mushrooms and cook, stirring, until the liquid evaporates.
Season the mushrooms with salt, pepper and nutmeg. Add the parsley.
Turn out the polenta and cut into 8 slices. Brush with oil.
Cook the polenta slices on a hot griddle brushed with olive oil until browned on both sides. Serve on plates with the mushrooms.