Add beef and peppers to a 4- to 6-quart slow cooker. Stir together hoisin sauce, orange juice, marmalade, soy sauce, garlic, sesame oil, 1/4 cup scallions, 1/4 cup cilantro, ginger, and hot pepper flakes (if using). Pour over beef and peppers; mix. Cover and cook until meat is tender, on high for 5 to 6 hours or low for 6 to 7 hours. Remove lid and skim fat. Transfer 1 cup liquid to a bowl; whisk in flour until smooth. Stir into slow cooker until combined. Cover with lid and cook on high for 30 minutes more or until slightly thickened.
Serve on top of rice. Garnish with remaining scallions and cilantro.