Trim any excess fat from the tenderloins and slice into thick slices (about 1-1/2 inch). Press each slice with your palm to flatten it slightly. Season with salt and pepper.
Heat the olive oil and 1 tablespoon butter in a large non-stick skillet until the butter is melted and foamy. Sear half the medallions until lightly browned, about 2 minutes on each side. Transfer to a plate and sear remaining meat. Remove the rest of the medallions to the plate while you prepare the sauce.
Add the remaining 2 tablespoons butter to the pan along with the shallots and a pinch of salt. Saute until the butter melts, then add the mushrooms and cook about three minutes.
Sprinkle in a tablespoon of flour, and stir well. Add the Marsala and chicken broth, and stir until bubbly.
Add the pork medallions and parsley and cook another 4-5 minutes or until the pork is firm to the touch and just slightly pink in the middle.