1. Preheat your oven to 400°F (200°C).
2. In a bowl, whisk together balsamic vinegar, olive oil, minced garlic, salt, and pepper.
3. Place the steak in a large zip-top bag and pour in the marinade. Seal the bag and refrigerate for at least 30 minutes or overnight for best results.
4. On a large sheet pan, arrange the zucchini, bell pepper, and red onion.
5. Remove the steak from the marinade, reserving the marinade, and place it on the sheet pan with the veggies.
6. Bake in the preheated oven for about 20-25 minutes, or until the steak reaches your desired level of doneness and the veggies are tender.
7. Drizzle the reserved marinade over the steak and veggies before serving.