01 -
Bring vegetable stock and milk to a gentle simmer in a heavy saucepan. Slowly whisk in the polenta in a steady stream to avoid lumps.
02 -
Reduce heat to low and cook for 20–30 minutes, stirring often, until the grains are tender and the mixture is creamy. Use the package timing for fine polenta; add a little hot water if it becomes stiff before tender.
03 -
Meanwhile heat oil and half the butter in a wide frying pan. Cook onion for 5 minutes, then add mushrooms. Cook for 10–12 minutes, stirring occasionally, until their moisture evaporates and they brown.
04 -
Add garlic and thyme to the mushrooms and cook for 1 minute. Season with half the salt and pepper.
05 -
Take the polenta off the heat and stir in the remaining butter, grated cheese and remaining salt and pepper. Taste; it should be soft enough to spread gently in the bowl.
06 -
Divide polenta among four shallow bowls and spoon the mushrooms and onion over the center. Serve immediately, since polenta firms as it cools.
07 -
Refrigerate cooled leftovers in covered shallow containers within two hours for up to three days. Reheat polenta with a splash of water or milk, stirring until hot and creamy; heat mushrooms thoroughly.