Cut chicken into 1 inch chunks and season with salt and pepper. Heat oil over medium heat in a large saucepan. Add shallots and saute about 3 minutes, or until they are a light golden color. Turn heat to medium high, add chicken, and cook until golden and almost cooked through, about 5 minutes. Transfer the chicken and shallots to a plate.
Turn heat to high and use Marsala to deglaze the pan, scraping up any browned bits from the bottom. Boil until liquid is reduced by half. Add chicken stock and boil until liquid starts to look syrupy. Reduce heat and pour any juices that have accumulated on the plate into the pan. Add the mustard and tarragon and simmer for 3 minutes. Turn off heat and whisk in sour cream until smooth. Turn heat to low and taste, add additional salt and pepper if needed.
Add chicken and shallots back to pan and warm through, do not allow to boil. Serve with noodles or rice.