Cook spaghetti in salted boiling water until al dente. Reserve ½ cup pasta water, drain pasta, and toss with butter.

Pat steak dry, season with salt, pepper, and soy sauce (plus cornstarch if using).

Heat olive oil in a large pan and sear steak cubes over high heat until browned and cooked to desired doneness. Remove from pan.

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.

Toss pasta into the sauce, loosening with reserved pasta water as needed. Add steak back to the pan and stir to coat.

Sprinkle with sesame seeds, parsley or green onions, and additional red pepper if desired.

Plate and serve immediately.