1) Preheat the oven to 220C/Gas 7.
2) Halve and core the pears and cut into 2-cm pieces. Heat 1 teaspoon of the olive oil in an ovenproof nonstick skillet over medium heat. Add the pears and cook, stirring occasionally, until beginning to brown, about 4 minutes. Transfer the pan to the oven and continue cooking until the pears have softened, about 5 to 7 minutes more.
3) Meanwhile, whisk the mustard and vinegar in a small bowl. Gradually whisk in the remaining olive oil, starting with a few drops and then adding the rest in a steady stream, to make a smooth, slightly thick dressing. Set aside.
4) Toss the watercress, rocket and chicory in a large bowl. Add the blue cheese and dressing, season with salt and pepper, and toss gently. Toss the warm pears into the salad.
Divide among four plates and serve immediately.