Heat a large pot over low heat and add the butter. Once melted, stir in the onions, salt, pepper and thyme. Cook, stirring often, until the onions are softened and are starting to caramelize, about 20 minutes. Stir in the red wine. Cook, stirring often again, for another 20 to 30 minutes until the onions are deeply caramelized and golden in color. Stir in the garlic.
Stir in the dry sherry and cook for about 5 minutes. Add in the beef stock and fresh thyme sprigs. Bring the mixture to a boil, then reduce it to a simmer and cook uncovered for 15 to 20 minutes, until the broth has reduced and thickened a bit.
While the soup is simmering, make your baguette slices. Preheat the oven to 350 degrees F. Place the slices on a baking sheet. Brush the bread with olive oil. Place in the oven for 10 to 12 minutes, until golden and crunchy.
To serve the soup, ladle spoonfuls of the soup into oven-safe ramekins or crocks. Place them on a baking sheet. Add a sprinkle of blue cheese on top, then a baguette slice. Pile more blue cheese on top. Repeat with remaining crocks. If you have leftover baguettes and blue cheese, I like to top the bread with the blue cheese and place them on a baking sheet too. These are great for extra dippers.
Turn the broiler on your oven to high with a rack in the center of the oven.
Stick the baking sheet in the oven. Broil for 2 to 3 minutes, or until the cheese is golden and bubbly. Be sure to keep an eye on them so the bread doesn’t burn! Garnish with extra thyme if you wish. And serve!