Slice the pork tenderloin into medallions about 1-inch thick. Season both sides with salt and pepper.
Heat olive oil in a large skillet over medium-high heat. Sear the pork on each side until golden brown, about 2–3 minutes per side. Remove and set aside.
In the same skillet, add butter and onion. Cook until the onion softens, about 3 minutes. Stir in garlic and cook for 30 seconds.
Add mushrooms and sauté until they release their juices and turn golden brown.
Pour in chicken broth, scraping up any browned bits from the pan. Let it simmer for 2 minutes.
Stir in cream, Dijon mustard, and thyme. Bring to a gentle simmer.
Return pork to the skillet, nestling the pieces into the sauce. Simmer for 5–7 minutes until pork is cooked through and sauce thickens.
Taste and adjust seasoning. Garnish with parsley and serve hot with rice, potatoes, or bread.