Bring a large pot of water to a boil.
Cook spaghetti according to package instructions.
Reserve 1 cup pasta water, then drain.
Heat a large skillet over medium-high heat.
Add ground beef and cook until browned, breaking it up with a spatula.
Add garlic, ginger, and red pepper to the pan.
Cook for 2–3 minutes until fragrant.
Pour in soy sauce and hoisin sauce.
Stir to combine and bring to a simmer.
Add reserved pasta water and red pepper flakes.
Stir and let simmer for a few minutes until slightly thickened.
Add cooked noodles to the pan and toss until fully coated in the sauce.
Add a splash of water if needed.
Stir in sliced green onions.
Taste and adjust seasoning if needed.
Serve in bowls and garnish with more green onions.
