Preheat oven to 450°F. Heat oil in a large skillet over high heat.
Season all sides of the pork tenderloin with rosemary, salt, and pepper, and add to the hot skillet, browning on all sides.
Transfer pork to a sheet pan, and bake for 12–15 minutes, or until a meat thermometer registers 145°F. Cover with aluminum foil, and let rest for 10 minutes before slicing thin.
Meanwhile, return the skillet you browned the pork in to the heat, and add the chicken stock to deglaze, scraping any browned bits from the bottom of the pan.
Add the Dijon mustard, garlic, and thyme to the skillet, and let the sauce reduce by about ⅓.
Remove sauce from heat, and stir in butter until slightly thickened.
Slice the pork tenderloin into 8-100 calorie slices. Serve each portion of the sliced pork drizzled with 1 ½ tablespoons of the sauce.
HELPFUL TIP: Whole-grain (sometimes labeled “coarse” or “deli”) mustard goes incredibly well in place of the Dijon in this sauce. It has a bit less spice than the Dijon if you want to keep things mild.