Bring a large pot of salted water to a boil. Cook the spaghetti according to package directions. Before draining, set aside 1 cup of pasta cooking water, then return the drained pasta to the pot.
While the pasta cooks, prepare the aromatics and vegetables. Thinly slice the onion, mince the garlic, and grate the ginger.
Heat 2 tablespoons of sesame oil in a large skillet over medium-high heat. Once hot, add the sliced onion and sauté, stirring frequently, until softened (about 4 minutes). Pour in the remaining 2 tablespoons of sesame oil and add the shaved beef. Let it sear briefly before stirring. Add the shredded carrots, grated ginger, and minced garlic. Stir everything together and cook until the beef is nearly done but still slightly pink, about 3 to 4 minutes. Push everything to one side of the skillet.
In the open space of the skillet, pour in ½ cup of the reserved pasta water, then add the soy sauce, rice vinegar, tahini, sriracha, honey, and fish sauce. Stir everything together until the sauce starts to blend, then mix it with the beef and vegetables. Let it simmer for another minute until slightly thickened.
Transfer the beef mixture into the pot with the cooked pasta. Toss everything together, adding small splashes of the reserved pasta water as needed to coat the noodles in a glossy sauce. Taste and adjust seasoning with salt if necessary.
Serve the noodles in bowls and garnish with any of the optional toppings for added texture and flavor. Enjoy!