Season the pork tenderloin generously with salt and black pepper on all sides.
Heat olive oil in a large Dutch oven over medium-high heat until shimmering.
Sear the pork tenderloin for 2-3 minutes per side until a brown crust forms, then transfer to a plate.
Add the diced onion and sliced mushrooms to the pot, sauteing for 5-7 minutes until the mushrooms release their moisture and brown.
Stir in the minced garlic and tomato paste, cooking for 60 seconds until fragrant.
Pour in the red wine to deglaze the pan, scraping the bottom with a wooden spoon to release the fond.
Add the beef stock, halved potatoes, rosemary, and thyme sprigs to the liquid.
Return the pork tenderloin to the pot, nesting it among the potatoes and mushrooms.
Reduce heat to low, cover with a tight-fitting lid, and braise for 25-30 minutes or until the internal temperature of the pork reaches 145 degrees Fahrenheit.
Remove the pork and let it rest for 10 minutes before slicing into medallions and serving with the reduced braising liquid and vegetables.