Heat sesame oil in a large 12-inch skillet over medium heat until shimmering, about 1 minute. Add sliced onion, white parts of green onions, minced garlic, and grated ginger. Sauté for 2-3 minutes, stirring frequently, until fragrant and onions turn translucent.

Add ground beef to the skillet, breaking it apart with a wooden spoon into small crumbles. Cook for 5-7 minutes, stirring occasionally, until completely browned with no pink remaining and you see a golden crust forming on some pieces. If using 85% lean beef or fattier, drain excess fat, leaving about 1 tablespoon.

Stir in sliced cabbage along with soy sauce, rice vinegar, hoisin sauce, and sriracha if using. Toss everything together thoroughly until cabbage is well coated with sauce. The cabbage will cook down to about half its original volume.

Continue cooking for 5-7 minutes, stirring every minute, until cabbage softens but maintains a slight crunch. The cabbage should be tender-crisp and vibrant green, not mushy or gray.

Taste and season with salt and black pepper as needed. Remember that soy sauce is already salty. Just before serving, sprinkle with reserved green onion tops and toasted sesame seeds.