Heat a large skillet over medium-high heat and add 2 teaspoons olive oil, shallots and garlic; cook until tender. Add mushrooms and cook 3 additional minutes; remove mushroom mixture from pan. Set aside.
Place each chicken breast half between 2 sheets of heavy-duty plastic wrap; pound to a 1/4-inch thickness using a meat mallet or rolling pin. Sprinkle both sides of chicken with 1/8 teaspoon salt and 1/8 teaspoon pepper. Place flour in a shallow dish; dredge chicken breast halves in flour.
Add butter to pan, and place over medium-high heat. Add chicken; cook until browned. Remove chicken from pan.
Add broth and Marsala to pan and scrape pan to loosen browned bits. Bring mixture to a boil and thicken with a flour water mixture (1 teaspoon of flour to 2 tablespoons water). Return chicken to pan and simmer for 5 to 10 minutes; add cream, 1/8 teaspoon salt, and 1/8 teaspoon pepper. Return mushroom mixture to pan and stir in peas. Cook until thoroughly heated.