To make the polenta, bring water and/or stock and a generous pinch of salt to a boil in a medium pot over medium-high heat.
Whisk the polenta into the boiling liquid. Whisk until smooth with no lumps, then reduce the heat.
Cover with a lid then allow to simmer very gently over low heat for 20-25 minutes or until the polenta has absorbed the stock, has thickened and is creamy.
Add the cream, butter and Parmesan cheese then season to taste. Cover with a lid and leave to steam off the heat for another 10-15 minutes. Stir well with a wooden spoon before serving.
Wipe the mushrooms clean with paper towel. Slice the mushrooms to your personal preference. Leaving some of the mushrooms whole will add interesting texture.
In a large pan set over medium high heat, melt the butter (or heat the oil) then cook the mushrooms in batches until golden brown. Season with salt and pepper then remove from the pan and set aside.
In the same pan, cook the garlic and sage until fragrant then pour in the cream.
Season with salt and black pepper then bring to a simmer. Cook for 5-7 minutes until the sauce coats the back of a spoon.
Add the mushrooms and a squeeze of lemon juice then simmer for another 5 minutes.
Spoon some of the polenta into serving bowls then top with the creamy mushrooms. Pour over the sauce and serve.