Wash and dry all produce. Preheat oven to 425 degrees. Halve potatoes. (TIP: Quarter any large ones.) Strip rosemary leaves from stems; discard stems. Finely chop leaves until you have 1 TBSP. Toss potatoes, rosemary, and a drizzle of oil on a baking sheet. Season with salt and pepper. Roast in oven until golden brown, about 25 minutes.

Halve, peel, and finely chop shallot. Pick tarragon leaves from stems and roughly chop until you have 1 TBSP; discard stems. Remove any large stems or ribs from half the kale (use the rest as you like), then finely chop. Pat steak dry with a paper towel.

Heat a drizzle of oil in a large pan over medium-high heat. Add kale and season with salt and pepper. Cook, tossing, until kale is wilted and tender, about 10 minutes (TIP: Add a splash of water to pan if you’re having trouble getting kale to wilt.) Remove pan from heat. Stir in sour cream. Season with salt and pepper and set aside.

Heat a drizzle of oil in a medium pan over medium-high heat. Season steak all over with salt and pepper. Add to pan and cook to desired doneness, 3-6 minutes per side. Remove from pan and set aside to rest. Reduce heat to medium-low.

Add shallot to same pan. Cook, tossing, until translucent, 1-2 minutes. Pour in vinegar, scraping up any brown bits. Add tarragon and cook until vinegar evaporates, about 1 minute. Stir in stock concentrate and 1/2 cup water. Bring to boil, then lower heat. Let simmer until reduced by half, 2-3 minutes. Remove from heat. Stir in 2 TBSP butter. Season with salt and pepper.

If kale has cooled, quickly reheat in pan over low heat. Slice steak against the grain. Divide potatoes, kale, and steak between plates. Spoon sauce over steak and serve.