For the polenta, bring the water, salt, and butter to a boil.
Slowly whisk in the corn flour. Bring the mixture back to a boil.
Simmer the polenta, stirring every few minutes while the water is absorbed, for 30 minutes, until the polenta is creamy and thick like porridge.
Begin preparing the mushrooms while the polenta is cooking. Heat the olive oil in a skillet over medium heat. Add the garlic and wait for it to darken without letting it burn.
Add the onion and mix it with a wooden spoon for about 4-5 minutes, until it’s golden.
Stir in the mushrooms and cover the pan with a lid. Let the mushrooms cook for 5 minutes.
Remove the lid from the pan and add the salt, pepper, parsley, and tomato paste. Mix everything well with a spoon and cover the pan. Simmer the mushrooms for 10 minutes, until soft. Taste the mushrooms and add salt and pepper, if needed.
Scoop the polenta onto a plate. Create a well in the middle of the polenta with a large spoon. Spoon mushrooms into the well. Garnish the dish with fresh parsley leaves.