If your sesame seeds are not pre-toasted, dry fry them in a skillet or frying pan until nicely browned but not burned. Divide in half and set aside.

Slice the beef sirloin into thin strips. Set aside.

In a bowl, combine the brown sugar, soy sauce, water, sesame oil, half of the roasted sesame seeds, ginger, garlic and flour. Add the beef strips and mix well to ensure the beef is well coated with the marinade. Let sit in the fridge for a few hours or overnight. If short on time, marinate for at least 1 hour.

Pour in the beef and its marinade into the slow cooker or crockpot. Cover and cook for 5-7 hours on low or until meat is tender. You can cook it for 3-4 hours on high, too.

Transfer the meat to a dish and sprinkle with the remaining sesame seeds and sliced green onions. Serve immediately with hot rice.