Place chicken breasts between two pieces of greaseproof paper and pound to 6mm thick with a meat mallet. Dust chicken with flour, salt, pepper and oregano.
In a frypan, saute 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.