In a mixing bowl, whisk together soy sauce, sesame oil, garlic, ginger, brown sugar, and black pepper to create the marinade.

Place the steak in a resealable bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour.

While the steak marinates, wash and chop mixed vegetables into bite-sized pieces.

Preheat the grill to medium-high heat, add vegetable oil to the grill grates to prevent sticking.

Remove the steak from the marinade and grill for 3-5 minutes per side for medium-rare doneness.

Once cooked, transfer the steak to a plate and allow to rest for 5 minutes.

Add the remaining oil and toss in the prepared mixed vegetables on the grill, cooking for 4-5 minutes.

Slice the rested steak against the grain into thin strips.

Combine the grilled steak and vegetables on a platter, drizzling any remaining marinade over the top.

Garnish with chopped green onions and sesame seeds before serving.