In a mixing bowl, whisk together soy sauce, olive oil, red wine vinegar, minced garlic, black pepper, and dried thyme.

Place the flat iron steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour, or up to overnight.

In a bowl, toss the sliced bell peppers and zucchini with olive oil, salt, and black pepper.

Preheat the grill to medium-high heat.

Remove the steak from the marinade and discard the marinade. Grill the steak for about 5-7 minutes on each side, or until it reaches your desired doneness (135°F for medium-rare).

During the last 5 minutes of grilling, add the vegetables to the grill and cook until tender and slightly charred.

Remove the steak from the grill and let it rest for 5 minutes before slicing against the grain.

Serve the sliced steak with the grilled vegetables on the side.