Cut the pork tenderloin(s) into 2" thick medallions and then flatten each medallion slightly with the palm of your hand; season both sides with salt and pepper.
Heat a few tablespoons of olive oil in a saute pan set over medium heat; When the pan is hot enough, add the pork medallions, 5 or 6 at a time, and cook them 2 to 3 minutes per side, until golden brown. Remove to a plate and repeat with the remaining meat; reserve.
Return the pan to the heat source and to it, add the sliced onion, garlic and mushrooms and cook, stirring often, until the vegetables are tender and fragrant, about 5 minutes.
Add the chicken stock, salt, pepper and herbes de Provence, stir well and bring to a simmer.
Meanwhile, combine the white wine, sour cream and cornstarch together in a small measuring cup and whisk until well combined; add that to the pan and stir well.
Return the medallions, along with their cooking juices, back into the pan; stir well and simmer for about a minute until the sauce thickens, then kill the heat.
Kill the heat and allow the meat to sit in the sauce for a few minutes, then stir in the chopped parsley and serve.