Step 1: In a large, heavy-bottomed pot or Dutch oven, melt the butter over medium-low heat. Add the thinly sliced onions and 1/2 teaspoon of salt. Cook, stirring frequently, for 30-45 minutes, or until the onions are deeply caramelized and a rich golden brown. Be patient, as this step develops the deep flavor.
Step 2: If using, add the dry white wine or sherry to the caramelized onions and stir, scraping up any browned bits from the bottom of the pot. Let it cook for 1-2 minutes until almost evaporated. Then, stir in the beef broth and fresh thyme. Bring to a gentle simmer and cook for 10-15 minutes, or until the liquid has slightly reduced and thickened. Taste and adjust seasoning with salt and pepper as needed. Set aside.
Step 3: Preheat your oven to 375°F (190°C). Arrange the baguette slices in a single layer on a baking sheet. Drizzle lightly with olive oil. Toast in the preheated oven for 5-7 minutes, or until lightly golden and crisp. This creates a sturdy base for your French Onion Soup Crostini Bites.
Step 4: Once the crostini are toasted, spoon a generous amount of the caramelized onion mixture onto each baguette slice. Top each with a good helping of shredded Gruyère cheese.
Step 5: Place the assembled French Onion Soup Crostini Bites back on the baking sheet. Bake for 8-12 minutes, or until the cheese is melted, bubbly, and golden brown. For an extra golden crust, you can briefly broil for 1-2 minutes (watch carefully to prevent burning).
Step 6: Let the French Onion Soup Crostini Bites cool for a minute or two. Garnish with freshly chopped chives or parsley before serving immediately. Enjoy these delicious French Onion Soup Crostini Bites!