In a small bowl, whisk together the soy sauce, brown sugar, pressed garlic, minced ginger, sesame oil, garlic powder, onion powder, and crushed red pepper flakes until combined.
In another small bowl, stir the cornstarch and 2 teaspoons water to make a thin slurry, then add it to the sauce and mix well.
Heat a large skillet over medium-high heat. Add the ground beef and diced onion, and cook, breaking the meat into pieces, until the beef is browned and no pink remains, about 6–8 minutes.
Pour the prepared sauce into the skillet with the beef, stir to combine, and bring to a simmer.
Simmer for about 5 minutes, stirring occasionally, until the sauce thickens and coats the beef.
Remove from heat and sprinkle with thinly sliced green onions; serve over rice, cauliflower rice, or in lettuce wraps if desired.