Wash and dry all produce. Preheat oven to 425 degrees. Halve potatoes lengthwise. Toss with 1 TBSP oil and 1/2 tsp Italian seasoning in a medium bowl (we’ll use more seasoning later). Season with salt and pepper. Arrange cut-side down on a baking sheet. Roast until browned and tender, about 25 minutes.

Halve tomatoes. Cut lemon into wedges. Pat steak dry with a paper towel and set aside to bring to room temperature.

In a small bowl, stir together sour cream, mayonnaise, mustard, and a pinch of horseradish powder. Season with salt, pepper, and more horseradish (to taste).

Heat a drizzle of oil in a large pan over medium-high heat. Season steak all over with salt, pepper, and another 1/2 tsp Italian seasoning (you’ll have some left over). Add to pan and cook to desired doneness, 4-8 minutes per side. Remove from pan and set aside to rest.

In a large bowl, toss together mixed greens, tomatoes, a drizzle of oil, and a squeeze of lemon. Season with salt and pepper.

Slice steak against the grain. Divide between plates along with salad and potatoes. Drizzle sauce over steak. Serve with any remaining lemon wedges on the side for squeezing over.