In a food processor, pulse the pecans with cornstarch, thyme, paprika, 1 and 1/4 teaspoon salt and 1/8 teaspoon cayenne until the nuts are chopped. Transfer the mixture to a medium bowl.
Whisk together egg and water in a small bowl. Dip each chicken breast into the egg mixture and then into the nut mixture.
Heat the oil over moderate heat in a large nonstick frying pan. Add the chicken to the pan and cook for 5 minutes. Turn and continue cooking until the chicken is golden brown and cooked through, 5-6 minutes longer.
Meanwhile, in a small bowl, combine the mayonnaise, mustard, vinegar, sugar, parsley, a pinch of cayenne and the remaining 1/4 teaspoon salt.
Serve with mustard dipping sauce. (Also pairs well with a creamy potato gratin or green beans.) The combination of the sweet pecans and the assertive mustard sauce lends itself to either a crisp sparkling wine or a Sauvignon Blanc.
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