Heat 1 tablespoon of the olive oil in a large skillet over medium high heat. Add the onion and sauté for 2 minutes, until tender. Add the radicchio and endive and sauté for 5 minutes until the lettuces begin to soften. Add the vinegar and sugar and sauté for another 3 or so minutes, until quite wilted.
Meanwhile, place the baguette slices on a baking sheet , brush with the remaining 2 tablespoons olive oil, and bake for 4 or 5 minutes, until lightly toasted. Remove from the oven and place on a serving platter.
Remove the lettuce mixture from the heat and spoon a bit onto each toasted baguette, and top with a hefty pinch of the grated cheese. Let the cheese melt a bit into the warm topping and serve warm or at room temperature.