In a mixing bowl, whisk together the maple syrup, Dijon mustard, olive oil, apple cider vinegar, minced garlic, salt, and pepper until well combined. The consistency should be smooth and slightly thick.
Place the turkey tenderloin in a zip-top bag or shallow dish. Pour the marinade over the turkey, making sure it’s thoroughly coated. Seal the bag or cover the dish and refrigerate for at least 30 minutes, preferably up to 2 hours, to let the flavors penetrate.
Heat a skillet over medium-high heat. Remove the turkey from the marinade, reserving the excess marinade. Sear the tenderloin on all sides for about 2-3 minutes per side until golden brown. This step locks in flavor and gives a beautiful caramelized crust.
Reduce the heat to medium-low. Pour the reserved marinade over the tenderloin in the skillet. Cover and cook for 10-12 minutes, turning occasionally, or transfer the skillet to a preheated oven at 375°F (190°C) and roast for about 15-20 minutes. Use a meat thermometer to check for an internal temperature of 165°F (74°C).
Remove the turkey tenderloin from heat and let it rest for 5 minutes before slicing. Garnish with fresh thyme if desired. Serve alongside your favorite sides for a complete meal.