In a large bowl, whisk together soy sauce, sesame oil, rice vinegar, Sriracha, honey, garlic, and ginger, then slowly whisk in oil until combined.
Place steak in a shallow baking dish and pour 1/3 of soy mixture over steak, then let sit 10.
Bring a large pot of water to a boil.
Cook soba noodles according to package directions, then drain and rinse with cold water.
In a large bowl, toss noodles with carrot, cucumber, 1/2 of the green onions, and sesame seeds.
Pour remaining soy mixture over noodles and toss gently to combine.
Heat a lightly oiled grill pan or grill over high.
Remove steak from marinade and season with salt and pepper (discard marinade).
Grill steak 5 per side for medium, then let rest 5 before thinly slicing on the diagonal.
Serve steak over soba noodle salad; drizzle with remaining 1 tablespoon sesame oil and garnish with remaining green onions.