Prep & Sear: Sprinkle chicken with 1/4 tsp salt and a little pepper, then coat with flour. Heat oil in a non-stick skillet and cook chicken, turning once, until browned; remove from pan.
Sauté Veggies: Add onion to the pan and cook until softened. Stir in any reserved flour and cook for 2 minutes. Add potatoes and the remaining salt and pepper, stirring for 3 minutes until hot.
Simmer: Stir in the broth and mustard, then return the chicken to the pan. Reduce heat to medium-low, cover, and simmer for 20 minutes until the chicken is cooked through.
Final Touch: Stir in the frozen peas and cook for 5 more minutes. Sprinkle with fresh parsley and serve.