Start a large pot of water on high heat to bring to a boil for the egg noodles.
In another large pot, cook the chicken breast until done. Remove the chicken and in the same pot add 2 tbsps of butter and cook down the onions and mushrooms.
Once the vegetables are lightly browned, create a well in the center of the pot. Melt the 4 tbsps of butter and add the flour to the butter to make a roux. Cook together for a couple of minutes before adding the chicken broth and cooked chicken. Stir everything together. Be sure to scrape the bits off of the bottom.
Add the egg noodles to the boiling water and cook until tender.
Just before the noodles are done, stir in the mustard and sour cream. Strain the noodles when done.
Serve the stroganoff over the noodles.