Heat oven to 375° F. Spread the walnuts on a rimmed baking sheet and toast, tossing occasionally, until fragrant, 8 to 10 minutes.
In a large bowl, combine the escarole, onion, Parmesan, and toasted walnuts.
In a small bowl, whisk together the vinegar, mustard, ½ teaspoon salt, and ¼ teaspoon pepper. Whisking constantly, gradually add the oil. Toss with the salad.