Pat the steaks dry with paper towels and season them generously on both sides with salt and freshly ground black pepper.

Heat the olive oil in a large cast-iron skillet over medium-high heat until the oil is shimmering and just starting to smoke.

Carefully place the steaks in the hot skillet and sear for 4-5 minutes on each side for medium-rare doneness, or until a golden-brown crust forms.

Remove the steaks from the pan and transfer them to a plate to rest loosely covered with foil while you prepare the sauce.

Reduce the heat to medium, add the minced garlic to the pan drippings, and sauté for about 30 seconds until fragrant.

Pour in the red wine to deglaze the pan, scraping up any browned bits from the bottom with a wooden spoon.

Add the beef broth, fresh blackberries, balsamic vinegar, and the sprig of rosemary to the skillet.

Simmer the mixture for 8-10 minutes, using the back of a spoon to gently crush the blackberries as they soften.

Once the sauce has thickened enough to coat the back of a spoon, remove the rosemary sprig and turn off the heat.

Stir in the cold unsalted butter until it melts completely, giving the sauce a glossy, velvety finish.

Slice the rested steaks against the grain or serve them whole, spooning the warm savory blackberry sauce generously over the top.