Wash and dry all produce. Adjust rack to middle position and preheat oven to 400 degrees. Halve, peel, and mince shallot. Remove and discard stems and large ribs from kale. Chop or tear leaves into 1-inch pieces. Cut potatoes into 3/4-inch-thick wedges. Using a heavy pan or mallet, pound peppercorns in their bag until coarsely ground.

Toss potatoes on a baking sheet with a drizzle of oil and a pinch of salt and pepper. Roast in oven until tender and browned, 30-35 minutes, tossing halfway through.

Meanwhile, melt 1 TBSP butter in a large pan over medium heat. Add kale and a splash of water. Cook, tossing occasionally, until leaves are wilted and tender, 4-5 minutes. Season with salt and pepper. Remove from heat, keeping kale in pan.

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, 4-7 minutes per side. Remove from pan and set aside to rest for a few minutes.

Heat another drizzle of oil in same pan. Add shallot and 1/4 tsp ground peppercorns (use more or less to taste). Cook, tossing, until shallot is soft, 2-3 minutes. Stir in stock concentrate and 1/2 cup water, scraping up any browned bits on bottom. Bring to a simmer and let bubble until reduced by half, 2-3 minutes. Remove pan from heat, then stir in half the sour cream.

Return pan with kale to medium heat. When kale is just warmed, remove pan from heat and stir in remaining sour cream. Season with salt and pepper. Slice steak against the grain, then divide between plates along with potatoes and kale. Drizzle steak with sauce and serve.