How to Make It
Preheat the oven to 350°. Spread the walnuts in a pie plate and toast in the oven for 8 minutes, or until golden and fragrant. Let cool, then coarsely chop.
In a small saucepan, combine the lemon juice with the chopped mint and cook over high heat just until the mint is wilted, about 1 minute. Remove from the heat and let steep for 10 minutes. Strain the lemon juice into a small bowl and discard the mint. Whisk the olive oil, vinegar and sugar into the lemon juice and season with salt and pepper.
Heat 1 tablespoon of the vegetable oil in each of 2 large skillets. Season the pounded chicken breast halves with salt and pepper, add 3 to each skillet and cook over moderately high heat until golden and just cooked through, about 3 minutes per side. Transfer the chicken to a cutting board and let cool. Thinly slice the chicken across the grain and transfer it to a large bowl.
Add the cantaloupe, feta and half the mint leaves to the chicken. Whisk the vinaigrette, add half to the chicken and toss gently. Put the watercress in another large bowl, add the remaining mint leaves and vinaigrette and toss gently to coat. Season with salt and pepper. Arrange the watercress on 6 plates or a large platter. Top with the chicken salad and serve immediately.