Add the milk, water and chicken stock to a large saucepan and bring to the boil. As soon as the water boils, pour in the polenta, whisking as you pour – a balloon whisk works best for this. (You will initially think there is far too much liquid, but the polenta is a super sponge and you want the polenta soft and velvety anyway.) Cover, turn heat to its lowest setting and cook for 10 minutes. Stir a few times.
Fry the mushrooms while the polenta is cooking. Start by adding the oil to the pan. As soon as it’s warm, add the butter, garlic and thyme and fry for 30 seconds. Add the chopped mushrooms. (For visual interest, I don’t chop them all the same. I slice, halve and quarter. The pretty littles ones, like shimeji, I leave whole.) If the pan gets too dry, add a bit more butter. Once the mushrooms are cooked, add the parsley, lemon juice and zest and stir through. Give it a big pinch of black pepper and salt to taste. The butter is salty, but you will need extra salt, remember salt is flavour!
Stir a tbs of butter and the parmesan into the polenta.I added a quearter teaspoon of salt as well, but first taste and add a bit at a time because your butter or stock may be saltier than mine. Dish the mushrooms on the polenta. For the pretty and the fresh add another generous pinch of parsley and grating of lemon zest to the mushrooms. Serve immediately with extra cheese on the side.