Bring a large pot of salted water to a boil. Cook the egg noodles according to package directions, about 8 to 10 minutes. Drain and set aside.

While the noodles cook, melt 2 tablespoons of butter in a large skillet over medium-high heat. Add the cubed chicken and season with 1/2 teaspoon salt and 1/2 teaspoon pepper. Cook for 6 to 7 minutes, turning occasionally, until golden and no longer pink in the center. Do not overcrowd the pan — cook in batches if needed. Remove from skillet and set aside.

In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the diced onions and cook until soft and translucent, about 5 minutes. Add the sliced mushrooms and cook for another 5 to 7 minutes until tender and lightly golden.

Sprinkle the flour over the vegetables and stir constantly for 1 to 2 minutes until the raw flour smell cooks off.

Pour in the chicken broth slowly, scraping up all the golden bits from the bottom of the pan.

Reduce heat to medium-low. Stir in the sour cream, Dijon mustard, cooked chicken, and the remaining 1/2 teaspoon each of salt and pepper. Simmer gently for 5 minutes, stirring occasionally, until the sauce thickens. Do not let it boil once the sour cream is added, as this can cause the sauce to separate.

Stir in the chopped fresh parsley and serve immediately over the warm egg noodles.