1. Preheat oven to 220°C (425°F). Heat olive oil in a large skillet. Add garlic paste and onion, and cook over medium-low heat for 15 minutes, stirring occasionally, until onion is soft. Add sugar, salt, and balsamic vinegar. Pour in beef broth and let simmer until liquid has evaporated, about 5 to 10 minutes. Add thyme leaves and remove skillet from heat. 
2. In the meantime, remove ends from the baguettes and cut into 16 slices. Place a spoonful of onion mixture onto each slice and sprinkle cheese on top. 
3. Bake crostini on a parchment paper-lined baking sheet for 10 minutes, until cheese is bubbling and golden. Serve immediately. 

Chef’s secret

The topping for these amazing appetizers can be prepared in the morning for later use. Try other Gruyère-style cheeses for the gratin.