Boil the chicken: Add 1.5 quarts water to a medium pot and bring to a boil over high heat. Once the water is boiling, add 18 oz chicken breast, 1 tsp whole peppercorns, 1 bay leaf, and 2 oz sliced onions. Cook for 12 minutes over high heat.
Cut the chicken: Once the chicken is cooked, remove it from the water (you can discard or keep the water for later use). Leave it to cool for at least 5 minutes. Then, cut the chicken into approximately 0.5-inch cubes. Transfer to a plate and set aside.
Sauté the mushrooms: Add 1 tbsp unsalted butter, 2 oz diced onions, and 6 oz button mushrooms to a cast-iron skillet. Cook for 4 minutes over medium heat or until golden brown.
Sauté the bell peppers: Add 4 oz diced red bell peppers to the same skillet. Cook and stir for an additional 2 minutes.
Add the seasonings: Add 4 oz frozen peas to the skillet and stir to combine. Then, add 1/2 tsp salt, 1/2 tsp black pepper, and 1 1/2 tbsp Dijon mustard. Stir to distribute the seasonings evenly.
Make a roux: Add 2 tbsp all-purpose flour to the skillet. Mix well so that the flour coats all of the ingredients.
Add liquids and reduce: Add 1 cup unsalted chicken broth, 4 tbsp heavy cream, and 1/2 cup whole milk. Add the cooked chicken back into the skillet and stir to combine. Simmer for 10 minutes over low heat, stirring occasionally, and turn off the heat.
Garnish and serve: Sprinkle 2 tbsp chopped fresh parsley on top of the chicken. Ladle the chicken and sauce into a bowl or deep dish and serve with 3/4 cup cooked rice per serving.