1. Sprinkle gelatin over 2 1/2 cups water in Dutch oven and let sit until gelatin softens, about 5 minutes. Adjust oven rack to middle position and heat oven to 300°F.

2. Heat softened gelatin over medium-high heat, stirring occasionally, until melted, 2 to 3 minutes. Stir in sherry, soy sauce, hoisin, molasses, scallion whites, ginger, garlic, five-spice powder, and pepper flakes. Stir in beef and bring to simmer. Remove pot from heat. Cover tightly with sheet of heavy-duty aluminum foil, then lid. Transfer to oven and cook until beef is tender, 2 to 2 1/2 hours, stirring halfway through cooking.

3. Using slotted spoon, transfer beef to cutting board. Strain sauce through fine-mesh strainer into fat separator. Wipe out pot with paper towels. Let liquid settle for 5 minutes, then return defatted liquid to now-empty pot. Cook liquid over medium-high heat, stirring occasionally, until thickened and reduced to 1 cup, 20 to 25 minutes.

4. While sauce reduces, using 2 forks, break beef into 1 1/2-inch pieces. Whisk cornstarch and remaining 1 tablespoon water together in small bowl.

5. Reduce heat to medium-low, whisk cornstarch mixture into reduced sauce, and cook until sauce is slightly thickened, about 1 minute. Return beef to sauce and stir to coat. Cover and cook, stirring occasionally, until beef is heated through, about 5 minutes. Sprinkle scallion greens over top. Serve.

Serves 6