Heat the butter in a medium saucepan, add the onion and sauté for 3-4 minutes, or until softened but not coloured.
Add the garlic and thyme and sauté for one minute. Add the mushrooms and sauté for 2-3 minutes. Stir in the parsley.
Transfer half of the mushroom mixture to a bowl.
Add the polenta and hot stock to the mushrooms left in the pan. Stir well and cook for five minutes over a medium heat, or until the polenta has absorbed the stock and become soft. Add the spinach to the polenta and stir well.
To serve, spoon the soft polenta into a serving dish. Top with the reserved mushrooms and garnish with chives.