Preheat the oven to 375℉. Heat the oil in a large heavy bottomed, oven safe skillet over high heat. Season the beef tenderloin with salt and pepper.
Add the beef tenderloin to the skillet and cook for 3-4 minutes on all sides. Remove the beef from the pan and set it aside. Reduce the heat to medium-low.
Add the onion and rosemary. Cook for 1 minute or until lightly golden, scraping up the bits from the bottom of the skillet as you stir.
Add the brown sugar, vinegar, mustard, and chopped peaches to the skillet. Stir to combine, then remove from heat.
Return the beef tenderloin to the pan then bake it in the preheated oven for 10 minutes or until it reaches an internal temperature of 145℉.
Let the beef rest for 5-10 minutes before slicing and serving topped with peach glaze from the skillet.