Slice pork tenderloin into medallions, about 1-inch thick.
Season both sides with salt and pepper.
Heat olive oil in a skillet over medium-high heat. Sear pork on both sides until golden, about 2-3 minutes per side. Remove and set aside.
In the same skillet, sauté onions until softened. Add garlic and cook for another minute.
Pour in chicken broth and scrape the bottom to release any browned bits.
Stir in heavy cream, Dijon mustard, and thyme. Let it simmer for 5 minutes until slightly thickened.
Return pork medallions to the skillet. Simmer gently for 10 minutes until pork is cooked through and sauce is creamy.
Garnish with fresh parsley and serve warm.