Remove tenderloin from packaging and pat dry. Rub with salt and olive oil. Place tenderloin in gallon ziploc bag (or glass container with lid if you avoid plastic).
Cut one grapefruit in half and squeeze the juice into the bag.
Mince garlic and remove thyme leaves from the springs. Add to bag and let marinade for at least 30 minutes.
When ready, preheat oven to 375.
In an oven safe dish, add tenderloin. Using the remaining 1 1/2 grapefruits, peel and remove plithe using just the meat of the fruit then surround the tenderloin in the pan.
Cover with foil and cook for 25-35 minutes (every oven is different) until there is no pink remaining inside of the tenderloin.
Remove from oven and enjoy!