Trim any fat from meat. Place the tenderloin in a large plastic bag set inside a bowl. Stir together the orange juice, water, green onion, sage, mustard, and fennel seed. Pour over meat in bag. Seal bag. Marinate for 30 minutes at room temperature or 4 hours in refrigerator.
Meanwhile, cut off and discard upper stalks of fennel bulbs, including feathery leaves (reserve leaves for garnish, if desired). Remove any wilted outer layers; cut off a thin slice from base. Wash. Cut fennel lengthwise into quarters. Cook fennel quarters and baby carrots in a covered saucepan in a small amount of boiling water about 10 minutes or until nearly tender. Drain and set aside.
Remove tenderloin from marinade. Reserve 1 cup marinade. (If necessary, add water to make 1 cup liquid.) Pat meat dry with paper towels. Place tenderloin on a rack in a shallow roasting pan. Roast, uncovered, in a 425 degree F oven for 15 minutes. Add cooked fennel and carrots to roasting pan. Roast 10 to 20 minutes more until an instant read thermometer registers 160 degree F. Cover meat with foil and let stand 5 minutes before carving.
Meanwhile, for sauce, in a small saucepan stir together reserved marinade and the cornstarch. Cook and stir over medium heat until thickened and bubbly. Cook and stir 2 minutes more. Remove from heat.
To serve, slice tenderloin. Serve with fennel and carrots. Spoon sauce over meat and vegetables. If desired, garnish with reserved fennel leaves. Makes 4 servings.