Cook noodles in salted boiling water until al dente, 9 to 10 minutes. Reserve ½ cup pasta water, then drain.
In a small bowl whisk soy sauce, broth, brown sugar, hoisin, cornstarch, sesame oil, red pepper flakes, and black pepper until smooth.
Set a large skillet over medium high heat. Add oil and ground beef. Cook 5 to 7 minutes, breaking it up, until browned. Drain extra grease if needed.
Add garlic, ginger, and most of the green onions. Cook about 30 seconds until fragrant.
Stir the sauce, then pour it into the skillet. Bring to a simmer 2 to 3 minutes until it thickens and coats the beef.
Add cooked noodles and toss with tongs until everything is coated and glossy. If it seems too thick, add a splash of pasta water.
Let sit off the heat 2 to 3 minutes so the sauce clings to the noodles.
Top with remaining green onions and serve hot.
