Heat the olive oil in a 10-inch skillet over medium heat. Add the onions and, using a wooden spoon, stir them well to coat in oil. Reduce heat to medium low and cook for 45 to 50 minutes, stirring frequently, until the onions turn a deep amber color.

Add the garlic, balsamic vinegar, herbes de Provence, salt, and pepper. Set aside and allow to cool slightly.

Preheat oven to 450°F.

Slice the baguette into one-inch rounds. Spoon caramelized onions onto each slice of bread and place enough cheese to cover the top without it hanging down the sides. Bake for three to four minutes, or until the cheese is melted and bubbly.

Garnish with additional herbes de Provence and serve hot.