Wash and dry all produce. Adjust rack to middle position and preheat oven to 425 degrees. Take out 2 TBSP butter and let sit at room temperature. Halve and peel shallot. Finely mince one half; thinly slice other. Halve tomatoes. Halve lemon and squeeze 1 TBSP juice into a small bowl. Strip 2 tsp thyme leaves from stems. Halve potatoes lengthwise.

Toss potatoes with a large drizzle of oil, stripped thyme, salt, and pepper on a baking sheet. Roast in oven until tender and lightly browned, 20-25 minutes.

Pick and finely chop enough rosemary leaves from stems to give you 1/2 tsp. Using a spoon, mix butter and 2 tsp minced shallot in another small bowl. (TIP: If butter is not soft, microwave on high for 10 seconds.) Stir in chopped rosemary. Season with salt and pepper. Place bowl in refrigerator and let chill until ready to serve.

Heat a large drizzle of oil in a large pan over medium-high heat. Pat steak dry with a paper towel, then season generously all over with salt and pepper. Add to pan and cook to desired doneness, 3-5 minutes per side. Remove from pan and set aside to rest.

Stir sour cream and 1 tsp water into lemon juice in bowl. (TIP: Add more water, 1 tsp at a time, to give mixture a drizzly consistency, if needed.) Season with salt and pepper. Place tomatoes and sliced shallot (to taste) in a medium bowl. Season with salt and pepper, then toss to combine. Halve romaine lengthwise, then cut into thick wedges.

Divide romaine between plates, then scatter tomato and shallot mixture on top. Drizzle with sour cream mixture. Slice steak against the grain, then add to plates, along with potatoes. Dollop steak with herb butter and serve.