Place stock and polenta in a medium-sized sauce pan, mix and bring to a boil
Reduce heat to low and you'll need to stir/whisk frequently
Add milk slowly
Add chopped garlic and salt
Add more stock, juice from the porcini water and milk if the polenta gets to thick. (should be creamy).
Cook and stir for about 30 minutes.
Make the mushroom and onion sauce:
Heat oil, then butter, in a medium-large skillet over medium heat
Add the onion, and still frequently for about 15 minutes
Once onion starts to brown, add the mushrooms, cook for another 15 minutes.
Add balsamic and red wine, cook down for about another 5 minutes
Add thyme, salt and pepper
Pour polenta into serving bowl
Place mushroom / onion mixture over the top
Garnish with extra thyme leaves