Preheat oven to 400°F (200°C).

On a large rimmed baking sheet, toss halved Brussels sprouts and cubed butternut squash with 2 tablespoons of olive oil, 0.5 teaspoon salt, and 0.25 teaspoon pepper.

Spread vegetables in a single layer and roast for 25-30 minutes, flipping once, until tender and caramelized.

Pat steaks dry with paper towels and season both sides with the remaining salt and pepper.

Heat a cast-iron skillet over medium-high heat with the remaining 1 tablespoon of olive oil until smoking.

Sear steaks for 3-4 minutes per side to achieve a medium-rare internal temperature.

Reduce heat to medium and add butter, minced garlic, and rosemary to the skillet.

Tilt the pan and continuously spoon the melting garlic butter over the steaks for 2 minutes.

Remove steaks from the skillet and let rest on a cutting board for 5 minutes.

Slice the steak against the grain and serve alongside the roasted vegetables, drizzling with the remaining pan butter.