Mix together 2 tablespoons mustard, the melted butter, 1/2 teaspoon salt and 1/8 teaspoon pepper. Brush mixture all over chicken; coat with ground pecans. Place on waxed-paper-lined baking sheet and refrigerate for 15 minutes.
Heat oil in large nonstick skillet over medium-high heat. Working in batches if necessary to avoid crowding, add chicken to skillet; sauté until browned, about 6 minutes per side (170°F). Remove skillet from heat; reduce heat to low. Remove chicken to platter and keep warm. Wipe out skillet with clean paper toweling.
Add sour cream and remaining mustard, salt and pepper to skillet, stirring until well blended. Gently heat through; do not let boil. Pour over chicken and serve immediately.