Sweet Vidalia French Onion Soup

3 medium sweet onions (Vidalia or Maui), peeled and sliced thin
1 teaspoon mustard seed
1 teaspoon celery seed
1 teaspoon freshly ground pepper
4 cups beef stock or consommé (homemade or any good quality organiic canned or boxed stock, such as Wolfgang Puck's)
1/4 cup red wine
1 T Worcestershire sauce
1 large portobello mushroom
4 slices of Gruyere or provolone cheese
olive oil (for brushing onto the mushroom)

Place all ingredients into a slow cooker. Cook on low for 4 hours.

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

Ladle the soup into ovenproof bowls. Place a mushroom slice on top and then a slice of Gruyere cheese. This soup is best when served in stoneware or other ovenproof bowls on a cookie sheet under the broiler for a minute or two and let the cheese get brown and bubbly.