Preheat the oven to 375 degrees Fahrenheit.
Using a sharp knife, make an approximately 1/4-inch deep slit down the length of the tenderloin(s), from one end to the other.
In a small bowl, mix the garlic with the sage, salt, pepper and 4 tablespoons of the olive oil. And spread this seasoning paste all over the meat, including inside of the slit.
Tie the meat with kitchen twine, or if you have two smaller pork tenderloin pieces, tie them together one on top of the other. To tie, cut a long string of kitchen twine and wrap it around the meat at one end, about 1-inch from the end. Tie a knot leaving two long ends, and with the remaining string, criss-cross over and around the meat down the length of the meat. Wrap around one more time at the other end and tie another knot. (Though no marinating time is necessary, you may cover and refrigerate for up to 24 hours in advance.)
Heat a large, ovenproof casserole or a deep, 12-inch skillet over medium-high heat. Add the remaining 2 tablespoons of olive oil. Once hot, add the tenderloin and brown on all sides, about 6 minutes total.
If you used a skillet to brown the meat, transfer it to an ovenproof casserole or baking dish. Add 3 cups of water and place in the oven. Roast for 30 minutes, or until the meat is cooked through and the internal temperature reads 150 to 160 degrees Fahrenheit. Remove from the oven.
When cool enough to handle, place the meat on a chopping board and cover with aluminum foil to keep warm. Pour the meat juices into a measuring cup and set aside.