Use a rolling pin to flatten the chicken to about 1/3# inch thick. A meat mallet works, too.
Slice the chicken into ⅕” strips; cut long pieces in half
Season lightly with salt and pepper
Heat the oil in an iron skillet (high heat)
Put the chicken into the pan and spread it so you can cook evenly
Let it brown on both sides
Turn the heat down to medium-high
Add the butter and onions stirring until the butter melts
Pour in the mushrooms, letting them fry until golden
Slowly add flour and stir for 1 minute
A little at a time, incorporate the broth
Add the peas, sour cream, and mustard
Let this simmer on a medium-low heat. The sauce should start thickening
Once it’s the consistency you want, remove it from the stove
Serve with egg noodles