Chicken Marsala

This is my favorite rendition of of chicken marsala.

24 oz baby bella mushrooms sliced
1 medium white onion finely chopped
2.5 lbs thinly sliced chicken breast
1 tbsp extra virgin olive oil
3/4 cup all-purpose flour
1 cup Riesling wine
1/2 cup marsala wine
1 can chicken broth
1 tbsp vegetable bullion
2 tbsp dry parsley
1/2 tsp black pepper
salt to taste