In a medium skillet, melt the butter over medium-high heat. Add the pears and sugar, reduce the heat slightly, and cook, stirring occasionally, until the pears are tender and slightly caramelized but still retain their shape, 5 to 6 minutes. Remove from the heat and cool on a plate.
In a medium bowl, whisk together the vinegars, shallots, and honey. Add the oil in a steady stream and whisk until emulsified. Season with salt and pepper.
In a large bowl, combine the radicchio, mesclun, watercress, and endive. Add dressing to taste to the greens and toss to coat.
Divide the greens among 4 large salad plates. Arrange the pear slices over each salad and top with the cheese and nuts. Drizzle lightly with additional dressing and freshly ground black pepper. Garnish the plates with the French bread toasts and serve immediately.