Boil a pot of salted water and cook the spaghetti according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the sesame oil over medium-high heat. Add the ground beef and cook until browned, breaking it apart with a spatula. Season with a pinch of salt and pepper.
Stir in the minced garlic and grated ginger, cooking for about 1 minute until fragrant.
Add the sliced bell pepper and julienned carrot to the skillet. Sauté for 3-4 minutes until the vegetables are tender-crisp.
In a small bowl, whisk together the soy sauce, hoisin sauce, rice vinegar, and chili paste (if using). Pour the sauce into the beef and veggie mixture, stirring well to coat.
Add the drained spaghetti into the beef and vegetable mixture. Using tongs, toss everything together until well combined, making sure the pasta is coated with the sauce.
Stir in the chopped green onions, and remove from heat. Plate the spaghetti, and garnish with sesame seeds and fresh cilantro.