Cook the Chicken: Heat olive oil in a large skillet over medium-high heat. Add the chicken pieces and cook until browned and cooked through, about 6-8 minutes. Remove the chicken from the skillet and set aside.
Sauté Vegetables: In the same skillet, add the chopped onion and cook until softened, about 3-4 minutes. Add the garlic and mushrooms (if using) and cook until the mushrooms are tender, about 4-5 minutes.
Prepare the Sauce: In a mixing bowl, whisk together the chicken broth, sour cream, flour, Dijon mustard, and paprika until smooth. Pour the mixture into the skillet with the onions and mushrooms. Bring to a simmer and cook until the sauce thickens, about 5-7 minutes.
Combine and Simmer: Return the cooked chicken to the skillet and stir to combine. Simmer for an additional 5 minutes, allowing the flavors to meld together. Season with salt and pepper to taste.
Serve: Serve the chicken stroganoff over cooked egg noodles and garnish with fresh parsley.