How to Make It
Preheat the oven to 450°. Bring a large saucepan of water to a boil. Add the pearl onions and blanch for 30 seconds; drain and let cool slightly. Peel the pearl onions and trim off the roots.
On a large rimmed baking sheet, toss the onions with 2 tablespoons of the olive oil and season with salt and pepper. Roast the onions for about 20 minutes, stirring once or twice, until they are caramelized and softened.
Meanwhile, brush a large baking sheet with 1 tablespoon of olive oil. Arrange the baguette slices on the sheet; brush with the remaining 3 tablespoons of olive oil. Sprinkle the Parmesan over the slices and bake for 7 minutes, or until the cheese is melted and the bread is crisp; transfer to a platter.
Sprinkle the onions with the vinegar; season with salt and pepper. Mound the onions on the cheese toasts and serve warm or at room temperature.