Preheat oven to 425°F (220°C).
In a large bowl, toss potatoes with 2 tablespoons olive oil, half the minced garlic, rosemary, 1/2 teaspoon salt, and 1/4 teaspoon pepper.
Spread potatoes onto a large baking sheet and roast for 10 minutes.
Meanwhile, season steak with remaining salt and pepper on both sides.
In a small bowl, mix remaining olive oil, balsamic vinegar, and remaining garlic.
Remove sheet pan from oven, push potatoes to one side, and add bell peppers and onions to the other side.
Place steak in the center of the pan, and brush with half of the balsamic mixture.
Return to oven and roast for 12-15 minutes for medium-rare steak (internal temperature 135°F), or adjust cooking time for desired doneness.
Remove from oven, spread softened butter over the steak, and let rest for 5 minutes.
Slice steak against the grain, drizzle with remaining balsamic mixture, and garnish with fresh parsley before serving.
