1/4 cup unsalted butter
6 yellow onions, sliced about 1/8 inch thick
1 Tbsp. brown sugar
2 Tbsp. balsamic vinegar
3 Tbsp. flour
1 bottle beer or 1 cup dry white wine
8 cups good-quality beef stock
1/2 tsp. freshly ground black pepper
1 tsp. dried thyme
1 tsp. salt
1 bay leaf
6-8 pieces French bread, sliced about 1-1/2” inch thick
6-8 slices Swiss or Gruyere cheese
1/2 cup grated Parmesan cheese
Combine butter, onions, brown sugar and balsamic in a slow cooker, cover and cook for 1 hour on high. Add flour and cook for 3 minutes, stirring occasionally. Add beer or wine, scraping any bits from bottom and side of slow cooker. Add stock, pepper, thyme, salt and bay leaf and cook on low for 8-10 hours.
To serve, preheat oven to 350 degrees and lightly toast bread slices. Increase heat to 450 degrees and place ovenproof bowls on a baking sheet. Ladle soup into bowls and top with toasted bread followed by Swiss and a sprinkle of Parmesan. Bake until cheese is melted and bubbly, about 10 minutes.
Makes 6-8 servings.