Slice the onions very thinly. Place a 5-quart stockpot over medium-high heat. Add the olive oil and the onions.

After about 10 minutes, lower heat to medium low. Add salt and pepper to taste, mustard seed, celery seed, and cracked pepper and continue to saute the onions until carmelized to a deep golden brown, another 20-25 minutes.

When the onions are a rich brown, add the beef stock, wine and Worcestershire sauce. Turn the heat up to medium and cook for about 15 minutes. Preheat the broiler.

In the meantime, cut the stem off the portobello mushroom. Place the mushroom on its side and slice into 4 thin pieces. Heat a large skillet over medium high heat. Brush mushroom slices with olive oil and place in skillet, cooking for a couple of minutes on each side until lightly browned.

Ladle the soup into 4 oven safe bowls. Place a mushroom slice on top and a slice of cheese. Place bowls on a cookie sheet and place under the broiler until cheese gets brown and bubbly.