Preheat oven to 425 degrees F.
Halve and core the pears and cut into 3/4-inch 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 pan to the oven and continue cooking until the pears are softened, about 5 to 7 minutes more.
Meanwhile, whisk the mustard and vinegar in a small bowl. Gradually whisk in the remaining 3 tablespoons 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.
Toss the watercress, arugula, and endive 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 4 plates and serve immediately.
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