Step 1: Begin caramelizing the onions. In a large, heavy-bottomed pot or Dutch oven, melt the butter with the olive oil over medium-low heat. Add the thinly sliced onions and sugar (if using). Stir well to coat.
Step 2: Cook the onions, stirring occasionally, for 45-60 minutes, or until they are deeply golden brown and very soft. Be patient and keep the heat low to prevent burning. This slow caramelization is key to the French Onion Soup Crostini Bites' flavor.
Step 3: Add the minced garlic to the caramelized onions and cook for another minute until fragrant. Pour in the dry sherry or white wine and scrape up any browned bits from the bottom of the pot. Cook for 2-3 minutes until most of the liquid has evaporated.
Step 4: Add the beef broth and fresh thyme sprig to the pot. Bring to a gentle simmer and cook for 15-20 minutes, allowing the flavors to meld and the liquid to slightly reduce. Remove the thyme sprig. Season with salt and pepper to taste.
Step 5: While the onion mixture simmers, preheat your oven to 375°F (190°C). Arrange the baguette slices on a baking sheet. Lightly brush each side with olive oil.
Step 6: Bake the baguette slices for 5-7 minutes, flipping halfway, until golden brown and crisp. Remove from oven and set aside.
Step 7: Preheat your broiler. Arrange the toasted crostini back on the baking sheet. Spoon a generous amount of the warm caramelized onion mixture onto each crostini. Top with a good sprinkle of grated Gruyère cheese.
Step 8: Broil for 2-4 minutes, watching carefully, until the cheese is bubbly and golden brown. Serve the French Onion Soup Crostini Bites immediately, garnished with fresh chives if desired.