In a medium bowl, whisk together the soy sauce, rice vinegar, sesame oil, sugars, chili sauce and black pepper. Taste and adjust ingredients to your liking.
Pour ¼ cup of the sauce in a shallow container or ziploc bag, add the steak and marinate in the refrigerator for at least 30 minutes. Set the remainder of the sauce aside.
After marinating, cook the spaghetti according to package instructions.
While the water is boiling, remove steak from marinade (discard liquid) and cook the steak in a pan over medium-high heat to desired doneness (no more than medium for best results).
Remove steak from heat and let sit for a few minutes before slicing thinly across the grain.
Drain the pasta well and transfer to a large bowl. Add the sesame sauce while noodles are warm and toss to combine.
Add green onions, sesame seeds and toss again. Top with sliced steak and cilantro. Also great served cold!