Juice two of the oranges and all of the limes into a bowl. Add the garlic to the bowl along with a quarter cup of freshly chopped cilantro. Whisk in your cumin, salt, pepper, and olive oil.
Pour the marinade into a plastic bag and add your tenderloin. Set aside in the fridge for six hours, (overnight if you can, but 6 hours will get you 90% of the way there).
Once the meat is done marinating, preheat your oven to 350 degrees F, and grab a cast iron pan, add a few ounces of vegetable oil and proceed to bring it up to medium-high heat on your stovetop.
Take your tenderloin and place it in your hot pan allowing it 60 seconds or so on a side before giving it a quarter turn. After the third turn remove from heat and slide in your probe thermometer.
Set the thermometer to 145 degrees F and move the whole pan to the oven. It should take about 30 - 45 minutes to reach the desired temperature Once done remove from the pan and allow to rest 15 minutes before cutting.