In a food processor, pulse the pecans with the cornstarch, thyme, paprika, 1 1/4 teaspoons of the salt, and 1/8 teaspoon cayenne until the nuts are chopped fine. Transfer the mixture to a medium bowl.
Whisk together the egg and the water in a small bowl. Dip each chicken breast into the egg mixture and then into the nut mixture.
In a large nonstick frying pan, heat the oil over moderate heat. 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 to 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 the chicken with the mustard dipping sauce.
Menu Suggestions: The crisp coating on the chicken invites a creamy potato gratin alongside. Green beans, perhaps sautéed in bacon fat, would taste great, too.
Wine Recommendation: The combination of the sweet pecans and the assertive mustard sauce lends itself to either a crisp sparkling wine or a stainless-steel-fermented sauvignon blanc from California.