Generously salt and pepper the pork butt on all sides. Heat a 5-quart Dutch oven over medium high heat until hot. Add the oil, and then put the roast in the pot. Leave undisturbed for a few minutes or until the meat is uniformly browned on that side. Turn the roast, browning the other sides in the same manner. When the pork is browned, transfer it to a bowl and set aside.

Add the garlic and mushrooms, and sauté until there is no mushroom liquid left and they are browned, about 10 minutes. Transfer the mushrooms to the bowl with the meat.

Turn down the heat to medium low and add the butter to the pot. Add the onions and sauté until sticky and medium brown (about 1 hour). Towards the end, you will need to stir frequently otherwise the onions will burn. When the onions are sticky, glossy and brown, add the wine and scrape all the browned bits off the bottom and sides of the pan. Preheat the oven to 250 degrees F.

When all the liquid has evaporated, add the stock, and return the meat and mushrooms to the pot along with the carrots, potatoes, salt and thyme. Cover with a tight fitting lid then place the pot in the oven.

After 3 hours, add the Cipollini Onions. Cover and braise for another 3 hours or until a fork effortlessly goes through the meat. Remove the meat and vegetables from the pot. If the sauce seems watery, you can reduce it by boiling. When it's the right consistency, whisk in the cream.

Carve the roast and serve it with the vegetables, covered in the mushroom onion sauce.