How to Make It
Preheat the oven to 400°. In a 9-by-13-inch baking dish, toss the tomatoes with 2 tablespoons of the olive oil and 1 teaspoon of the sugar; season with salt and pepper. Roast for 35 to 40 minutes, shaking the dish occasionally, until the tomatoes start to burst and are lightly browned. Leave the oven on.
Meanwhile, in a large skillet, melt the butter in the remaining 1 tablespoon of oil. Add the onions and the remaining 1/2 teaspoon of sugar; season with salt and pepper. Cook over moderately high heat, stirring occasionally, until the onions start to brown, about 12 minutes. Reduce the heat to moderately low and cook, stirring occasionally, until the onions are very soft, 20 to 25 minutes longer.
Brush the baguette slices with olive oil and arrange on a large baking sheet. Bake for about 12 minutes, until lightly golden but still chewy; let cool slightly. To serve, top the crostini with the cheese, onions, tomatoes and any accumulated tomato juices. make ahead The roasted tomatoes and caramelized onions can be refrigerated separately overnight. Bring to room temperature before serving.