Place all the soup ingredients in the pressure cooker, except for the chicken. Stir to combine, then place the chicken breasts on top.
Close the lid. Select pressure cook/manual and high pressure [I have a a different brand of pressure cooker and select the "chicken" setting]; set time for 20 minutes. After the time is up, allow the pressure cooker to do a natural release for at least 10 minutes (see notes).
Meanwhile, make the roux. Melt the butter in a small saucepan, and whisk in the flour. Cook for about 5-7 minutes, whisking constantly. The butter and flour mixture will bubble and thicken.
Slowly add the milk to the flour mixture, whisking constantly. Cook for a minute or two and remove from heat.
Once the soup is done, remove the bay leaves. Remove the chicken, shred, and return to the pot. Add the roux and stir. Serve with fresh parsley, if desired.