Preheat oven to 350°.

Cut tenderloin lengthwise, stopping about 1/2-inch from cutting through. Open the split and flatten the meat out.

Sauté the onions, garlic, celery, carrot, and seasonings in butter until they are tender. Add 1 1/2 cups cooked wild rice; stir to combine. Spread stuffing mixture over split meat. Fold meat back over itself. Tie with kitchen string. Place the tied tenderloin in a shallow roasting pan and add chicken broth.

Bake at 350° until meat reaches an internal temperature of 160°, about 1 hour. Pour broth into a sauce pan and skim off excess fat. Combine the cornstarch with a few teaspoons of broth and stir to dissolve. Stir the cornstarch into the broth; bring the sauce to a boil and then reduce the heat to medium.  Cook until thickened.