Bring a large pot of water to boil. Prepare spaghetti as per the instructions on the package.
Meanwhile, in a small bowl, combine sauce ingredients and set aside.
In a wok or large frying pan on medium high heat, add cooking oil. Add the beef. Cook for one minute to sear one side. Flip, sear the beef on the other side. Then stir fry and cook until the beef is evenly browned. Remove the meat or push it to the sides.
Turn the heat down to medium, add garlic, ginger and chili flakes if using, cook for 30 seconds, until fragrant. Add the sauce and let simmer for about 3-5 minutes. In the same bowl, dissolve cornstarch in cool water to make a slurry. Add the slurry to the sauce and cook until the sauce starts to simmer and thicken. Stir in the ground beef.
Add the cooked and drained spaghetti and combine well. Cook for 1 minute. Turn off the heat.
Serve on a platter and garnish with sesame seeds.