Mix flour pepper, garlic salt, and oregano in a gallon size zip lock bag and add chicken breast, 2 at a time. Shake to coat well.
In a large skillet, melt butter over medium high heat. Add chicken and cook until brown on each side, about 5-6 minutes per side. Once cooked transfer to a rimmed baking sheet and cover with foil to keep warm while making sauce.
For the sauce:
In the same skillet over medium heat add: garlic, mushrooms, onions, marsala wine, chicken broth, and heavy creamy. Whisk to combine.
Remove 1/4 cup of sauce and mix it with cornstarch. Add it back into the skillet and whisk again to combine and thicken the sauce. Allow sauce to simmer for 10 minutes until heated through and thickened.
Add the chicken to the sauce and garnish with sage, parsley, and lemon slices. Serve hot over mashed potatoes or pasta.