Heat a skillet to medium-low. Add the butter, olive oil, garlic and onions. Saute until the onions are softened. Add the mushrooms and sherry. Season with salt and pepper. Cook until the liquid is incorporated. Let cool. Spoon out any extra liquid (if there is any) and discard.
Slice the brie right down the middle (horizontally) to make 2 rounds. Spread the mushroom mixture on one of the rounds. Place the other round on top of the mushrooms.
Lightly roll out the puff pastry. Place the stuffed brie on the pastry. Fold the pastry around the brie and wrap around all sides. Turn the brie over so the smooth side is on top. Mark the sides of the wrapped brie with vertical fork lines. Place in a greased pie plate and bake in a preheated 450 degree oven for 10 minutes. Turn down to 400 degrees, and continue to bake until golden brown. Watch closely... if overcooked, the brie will leak.
Remove from the oven and serve immediately. Serve with baguette slices.