How to Make It
Heat the oil in a large skillet (preferably cast iron) over medium high. Season the steak with 1 teaspoon salt and ½ teaspoon pepper. Add to skillet and cook, turning once, until deeply browned, about 3 minutes per side for medium-rare. Transfer to a cutting board and let rest 10 minutes before slicing.
Meanwhile, reduce heat to medium. Add the cabbage wedges to the skillet, cut-side down, and scatter the tomatoes around them. Cook, turning once, until the cabbage is lightly browned and the tomatoes begin to burst, about 4 minutes per side.
Whisk the mayonnaise, buttermilk, and vinegar in a small bowl and season with the remaining ¼ teaspoon each of salt and pepper. Drizzle the dressing over the cabbage and serve with the steak and tomatoes, with the shallot on top.