Preheat your oven to 425°F (220°C).
On a cutting board, season the flank steak with salt, pepper, minced garlic, and rosemary. Rub in the seasonings to ensure the steak is well-coated.
In a mixing bowl, combine the halved baby potatoes, green beans, and sliced red bell pepper. Drizzle with olive oil, salt, and pepper, and toss to coat the vegetables evenly.
Spread the seasoned vegetables on one side of the sheet pan in a single layer.
Place the seasoned flank steak on the other side of the sheet pan.
Transfer the sheet pan to the preheated oven and roast for about 20 minutes or until the steak reaches your desired level of doneness (use a meat thermometer for accuracy: 135°F for medium-rare).
Once cooked, remove the sheet pan from the oven and allow the steak to rest for 5 minutes before slicing.
Slice the steak against the grain and serve alongside the roasted vegetables.