Cook the Pasta: Bring salted water to a boil and cook spaghetti until al dente. Reserve ½ cup pasta water, drain pasta, and toss with butter.
Prepare the Steak: Pat steak dry, season with salt, pepper, and soy sauce (plus cornstarch if using).
Sear the Steak: Heat olive oil in a large pan and sear steak cubes over high heat until browned and cooked to desired doneness. Remove from pan.
Make the Sauce: In the same pan, sauté garlic until fragrant. Add soy sauce, honey, brown sugar, sriracha, vinegar, sesame oil, crushed red pepper, and water. Simmer to thicken.
Combine: Toss pasta into the sauce, loosening with reserved pasta water as needed. Add the steak back to the pan and stir to coat.
Garnish: Sprinkle with sesame seeds, parsley or green onions, and additional red pepper if desired.
Notes
Adjust the level of spiciness by adding more or less sriracha and red pepper flakes to suit your taste.