Do not wash or wipe the skillet. Place it back over medium heat. Add the butter, when it’s melted add the mushrooms.
Cook the mushrooms until the water has cooked out and they start to get golden in spots. This will take about 7-10 minutes. Stir occasionally.
Sprinkle the flour over the mushrooms and stir well until combined. Cook the mushroom mixture for 1-2 minutes to cook the flour a bit. Stir almost constantly.
Add in the onion soup and the water. Stir well, while gently scrapping any bits off the bottom of the skillet.
When the mushroom sauce comes to a simmer, add in the reserved diced chicken and stir well.
Once the sauce is simmering, lower the heat to medium-low, cover and cook approximately 20 minutes until the sauce thickens, and the flavors come together. Stir occasionally and keep it at a simmer.
Cook the egg noodles to package directions; use the lowest suggested cooking time. Remember to salt the pasta water.
Drain the noodles, but before doing so, reserve about 1 cup of the cooking water to loosen the sauce if needed.