Heat a large wok or skillet over high heat and add the sesame seed oil and vegetable oil. Once the oils are hot, saute the garlic for 20 seconds and add the ground beef. Cook until the beef is no longer pink.
Add the shredded cabbage, half of the sliced green onions, and red bell pepper slices. Cook until the veggies start to wilt, about 5 minutes.
Mix in the soy sauce, rice vinegar, black pepper, red pepper flakes, and salt if needed. Cook for about 3 minutes.
In a small bowl, mix the cornstarch with ½ cup water. Pour it over the beef and cook until the sauce has thickened. Serve over freshly cooked white rice and with the remaining sliced green onions on top.