Pat the bavette steak dry thoroughly with paper towels. This is crucial for a good sear. Season generously all over with sea salt and black pepper.

Heat olive oil in a large cast-iron skillet or heavy-bottomed pan over medium-high heat until shimmering and almost smoking. Carefully place the seasoned bavette steak in the hot skillet.

Sear for 4-6 minutes per side for medium-rare (internal temperature 130-135°F / 54-57°C), or longer for desired doneness. Bavette is best served medium-rare to medium to keep it tender. Use tongs to flip the steak.

Once cooked to your liking, transfer the steak to a cutting board, tent loosely with foil, and let it rest for at least 10 minutes. This allows the juices to redistribute, ensuring a tender and flavorful steak.

While the steak is resting, reduce the heat to medium-low and add butter to the same skillet (do not clean it, the browned bits add flavor). Add the thinly sliced shallots and sauté until softened and translucent, about 3-4 minutes.

Pour in the red wine, scraping up any browned bits from the bottom of the pan with a wooden spoon. Let it simmer for 2-3 minutes until slightly reduced.

Add the beef broth, cranberries, chopped rosemary, and balsamic vinegar (if using). Bring the sauce to a simmer and cook for 5-7 minutes, or until the cranberries begin to soften and burst, and the sauce slightly thickens. Stir in the granulated sugar, adjusting to your preferred level of sweetness and tartness.

Taste the sauce and season with salt and freshly ground black pepper as needed.

Slice the rested bavette steak against the grain into thin strips. Arrange the sliced steak on serving plates and spoon the warm cranberry shallot sauce generously over the top. Serve immediately with your favorite sides like roasted potatoes or green beans.