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.
Notes
The sweet balsamic glaze is a great counter note to the bitterness of the lettuces. You can make the sautéed mixture ahead of time, then gently reheat it before using it to top the toasted baguette, and finishing off the whole thing with the cheese.