Preheat your oven to 400°F (200°C).

In a large oven-safe skillet, heat 2 tablespoons of olive oil over medium heat. Add the cipollini onions and season with salt and pepper. Sauté until they are golden brown and caramelized, about 8-10 minutes. Remove from the skillet and set aside.

Season both sides of the T-Bone steaks generously with salt and pepper. In the same skillet, add the remaining 2 tablespoons of olive oil and increase the heat to high. Once the oil is hot, add the steaks and sear for 3-4 minutes on each side, until a nice crust forms.

While the steaks are searing, in a small bowl, combine the mashed black garlic, softened butter, chopped rosemary, and balsamic vinegar. Mix until well combined.

After searing the steaks, remove the skillet from the heat. Spread the black garlic confit butter mixture over the top of the steaks. Scatter the caramelized cipollini onions around the steaks and sprinkle with fresh oregano.

Transfer the skillet to the preheated oven and roast for 10-15 minutes, or until the steaks reach your desired level of doneness (130°F for medium-rare, 145°F for medium).

Once cooked, remove the skillet from the oven. Let the steaks rest for 5 minutes before serving.

Serve the T-Bone steaks with the roasted cipollini onions and any pan juices drizzled over the top.