- In a non-metal casserole dish, combine grapefruit juice, red onion, garlic, sugar, thyme and oregano. Add pork tenderloin, cover and marinate 1 hour, turning pork once to ensure that it marinates evenly.
- Preheat oven to 425°F. Transfer pork tenderloin with onions and herb mixture to a roasting pan, reserving liquid from marinade. Place in oven and roast 20 minutes. Reduce heat to 325°F and continue roasting for 1 ½ hours or until internal temperature reaches 145°F, basting occasionally in the first 30 minutes of cook time with 1/2 cup of marinade.
- Remove pork from pan and allow to rest for 3 minutes. Reduce oven temperature to 375°F.
- While pork rests, add remaining marinade to roasting pan, stirring until pan drippings loosen. Transfer mixture to small saucepan and bring to boil, uncovered. Reduce heat and simmer 5-10 minutes.
- Cut grapefruit in half and around edges. Sprinkle each half with brown sugar and bake in oven for 5-7 minutes. After 5 minutes, turn on broiler and broil until sugar has caramelized.
- Slice pork and serve with cooked marinade and broiled grapefruit segments.
per serving: 210 calories, 3g fat, 1g saturated fat, 95mg cholesterol, 80mg sodium, 14g carbohydrate, 1g fiber, 12g sugars, 31g protein