1. In a pan over low heat, heat oil and salt. Add onions, sugar and balsamic vinegar. Cook slowly, stirring occasionally until the onions are tender, translucent and a deep golden color. Depending on the type of onion (as they all caramelize at different rates based on their natural sugar content) this may take up to 40 minutes.
2. While the onions caramelize, heat oil in a skillet over medium-high heat.
3. In a mixing bowl, toss pear slices, brown sugar and cinnamon. Add pears to the skillet and cook until just tender. Remove from heat.
4. Preheat oven to 350 degrees.
5. Onto a baking sheet, lay the sliced baguette. Top with a small spoonful of the caramelized onions, a spoonful of the caramelized pears, and a thin slice of cheese. Bake for approximately 5 minutes or until cheese has melted.
6. Serve warm.
To save time on the day you're serving the dish, caramelize the onions and pears 1 or 2 days ahead of time and store in airtight containers in the refrigerator. Then quickly assemble the crostini just before serving.