Place the mushrooms on a baking sheet in a single layer and roast in a preheated, 400 degree oven until caramelized, for about 20min, turning them once halfway through.
As the mushrooms are cooking, melt the butter in a pan over medium heat. Add the onions and cook until tender (about 5-7 minutes).
Add the garlic and thyme and cook for about a minute.
Add the flour and cook for 2 minutes.
Once the mushrooms and onions are cooked, place in the crock pot with the wine and chicken stock. Add brie.
Cook on low for 6-10 hours or on high for 4-6 hours.