Toss sliced onions with melted butter, oil and salt and place them in a lightly greased slow cooker.
Cook on low and let the the onions caramelize over night, 10-12 hours.
In the morning, throw in the thyme, minced garlic, red wine, balsamic vinegar, beef stock, chicken stock and S&P to taste.
Cook on low for 8-10 hours.
Preheat the oven broiler.
Arrange the bread slices in a single layer on a baking sheet and broil 2 minutes, turning once until lightly toasted on both sides.
Remove from the heat; do not turn off the broiler.
Arrange oven-proof soup bowls on a rimmed baking sheet.
Fill each bowl ⅔ full of soup.
Place the toasted bread slices on top of the soup.
Sprinkle the bread slices with 2 Tablespoons each of the Parmesan and Gruyère cheeses.
Broil 5 minutes or until bread and cheese are golden brown.