Trim all the fat from the pork tenderloins and set aside.

Place the soy sauce, sherry, olive oil, brown sugar, garlic, onion and cinnamon in a large resealable bag and mix well. Add the pork tenderloins, seal, massage really well and store in the refrigerator for at least 4 hours, preferably overnight.

Preheat oven at 375ºF (190ºC).

Take out the pork tenderloins from the bag and reserve the marinade. Sear them on both sides in a large pan over medium high heat, for about 4 minutes per side. Then, place them in a roasting pan covered with aluminum foil and roast them in the oven for about 25-30 minutes or until a thermometer inserted in the thickest part of the meat reaches 150ºF - 155ºF (65.5 - 68ºC). Let the pork rest for about 10 minutes before slicing.

While the pork is roasting, bring the reserved marinade to a boil in a small sauce pan and simmer for about 5-10 minutes on low heat. During the last minute or so, dissolve the cornstarch in the 2 tbsp of water and add it to the marinade. Stir until it thickens and serve along with the pork.