Add 2-1/2 cups water to Dutch oven. Sprinkle gelatin over water and allow gelatin to soften for 5 minutes. Set a oven rack to the middle of oven and pre-heat oven to 300-degrees. Prepare the scallions by separating the white and green parts. Slice the green parts thinly on a bias, and smash the white part. Peeled the ginger, cut in half lengthwise, and crushed. Peel the garlic and smash.
Set Dutch oven over medium-high burner and heat for 2 to 3 minutes until the gelatin has melted. Add sherry, soy sauce, hoisin, molasses, scallion whites, ginger, garlic, five-spice powder, and pepper flakes. Then add beef, stir, and bring up to simmer.
Remove Dutch oven from heat. Cover tightly with heavy-duty aluminum foil, then top with the lid. Put in pre-heated oven and cook until beef becomes tender; 2 hours. Stir halfway through cooking time.
Use a slotted spoon to remove beef to a cutting board. Strain sauce through fine-mesh strainer into a fat separator. Use paper towels to wipe out Dutch oven. Allow liquid to settle for 5 minutes, then return defatted juices to now-empty pot. Reduce liquid over medium-high burner, stirring occasionally, until reduced to 1 cup; 20 to 25 minutes; stirring constantly.
Meanwhile while the sauce reduces, break beef into 1 1/2-inch pieces using two forks. In a small bowl, add cornstarch and 1 tablespoon water and whisk together until combined.
Turn down burner to medium-low, re-whisk cornstarch, add into pot and cook for 1 minutes. Add beef back to pot, stir to cover beef. Cover and allow to cook for 5 minutes, stirring occasionally, until beef is hot. Serve sprinkled with scallion greens.