Using a small whisk, make a slurry with the cornstarch and about 1/4 cup of the reserved juice. Combine the slurry with with the remaining 1/2 cup grapefruit juice. Place the mix in a microwave safe container and microwave/cook on low heat for about 3-4 minutes, stopping when you see the mix beginning to thicken. Add the reserved grapefruit sections and herbs, heat for about 30 seconds on low heat and stir to combine. Use this to sauce some pre-cooked pork tenderloin slices that have been slowly warmed in your microwave. It will also work with well as a sauce for poultry or seafood, such as shrimp. Serve the meat with the sauce on the side.