1. For the rosemary salt, combine chopped rosemary and salt on a cutting board and chop together. Stir in pepper; transfer to a small bowl, and set aside.

2. For the mashed ‘potatoes’, place the cauliflower in a steamer basket and set aside. Place potatoes in a saucepan large enough to fit steamer basket, add cold salted water to cover by about 2 inches, bring to a boil, reduce heat. Set steamer basket over boiling water, cover, and steam 15 minutes while simmering potatoes until tender.

3. Transfer cauliflower to a food processor, add the buttermilk, and ¼ cup of the milk, and puree until very smooth, about 2 minutes.

4. Meanwhile, drain potatoes, return to the saucepan, and heat over medium heat for 1 to 2 minutes to dry. Mash with a potato masher. Add the cauliflower puree, ¼ cup milk, 1 teaspoon salt, and ¼ teaspoon pepper. Stir to blend, and heat over medium-low heat, adding more milk as needed for a creamy consistency. Stir in the butter and the scallions. Cover, and set aside to keep warm.

5. Heat a grill or grill pan. Sprinkle the steak on both sides generously with the rosemary salt, pressing it into the meat. Grill the steak 4 minutes on one side. Turn, and grill 3 to 4 more minutes. Remove to a plate and let stand 5 minutes. Thinly slice the beef against the grain. Add the vinegar to the pan and stir to incorporate any pan juices.

6. To serve, make a bed of lettuce on each of four plates. Arrange one-quarter of the steak slices on top of the lettuce. Spoon on some of the cauliflower mashed potatoes. Scatter tomatoes all over, drizzle with the balsamic-meat juices, and garnish with chopped mint.