Place Chicken Breasts Between Two Pieces Of Wax Paper, And Pound To 1/4 Inch Thick With A Meat Mallet. Dust Chicken With Flour, Salt , Pepper And Oregano. In A Skillet, Fry Chicken In Olive Oil Over Medium Heat. Cook Until Done, Turning To Cook Evenly. Set Aside, And Keep Warm. In The Same Pan, Melt The Butter Over Medium Heat; Add Mushrooms, Sun-Dried Tomatoes, And Marsala Wine. Cook For Approximately 10 Minutes, Stirring Occasionally. Mix In Spinach, And Cook For About 2 Minutes. Serve Over Chicken.