Season the pork fillet generously all over with salt and pepper. Crush the garlic cloves with the flat side of a knife.
Heat the olive oil in an oven-safe pan over medium-high heat (375°F/190°C). Add the pork fillet, crushed garlic, and thyme. Sear the pork for 2-3 minutes per side until golden brown all over.
Transfer the entire pan to a 400°F/204°C oven and roast for 12-15 minutes, or until the internal temperature reaches 145°F/63°C.
Remove the pork from the pan and let it rest on a cutting board, tented with foil to keep warm, for 5 minutes.
Place the pan back on the stovetop over medium heat. Add the Pedro Ximenez and cook, stirring to scrape up any brown bits, until reduced by half.
Add the stock and continue cooking until the sauce reduces and becomes thick enough to coat the back of a spoon. Whisk in the butter until melted and glossy.
Slice the pork evenly into medallions and arrange on plates. Pour the sauce over and serve.
