Preheat the oven to 350°. In a large bowl, combine the lemon juice and minced shallot and let stand for 10 minutes. Gradually whisk in the olive oil and season the dressing with salt and pepper.
Meanwhile, spread the walnut halves in a pie plate and toast in the oven until golden brown and fragrant, about 6 minutes. Let the walnuts cool, transfer to a work surface and coarsely chop them.
Lightly season the avocado wedges with salt. Add the avocado, chopped toasted walnuts and watercress to the dressing and toss well. Season the salad with salt and pepper and serve right away.