Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce. Dice potatoes into 1/2-inch cubes. Halve, peel, and mince shallot until you have 2 TBSP (save the rest for another use). Thinly slice chives. Place peppercorns and mustard seeds in separate zip-close bags. Using a heavy pan or mallet, pound peppercorns and mustard seeds until coarsely ground.

Place potatoes in a medium pot with enough salted water to cover by 2 inches. Boil until tender when pierced with a fork, about 15 minutes. Drain and return to pot. Mash with a potato masher or fork until smooth. Stir in half the sour cream, half the chives, salt, and pepper. Cover and keep on very low heat, stirring occasionally, until ready to serve.

While potatoes cook, toss broccoli on a baking sheet with a large drizzle of olive oil, salt, and pepper. Roast on top rack until browned and tender, about 15 minutes.

While broccoli roasts, pat steak dry with paper towels; season with salt and pepper. Heat a drizzle of oil in a large pan over medium-high heat (use a nonstick pan if you have one). Add steak and cook to desired doneness, 3-6 minutes per side. Turn off heat; transfer to a cutting board. Rinse and wipe out pan.

Melt 1 TBSP butter in same pan over medium heat. Add minced shallot and cook until softened, 1-2 minutes. Add stock concentrate, 1/4 cup water, and peppercorns and mustard seeds (start with 1/2 tsp each). Simmer until reduced, 2-3 minutes. Stir in any resting juices from steak. Turn off heat; stir in 1 TBSP butter and remaining sour cream until smooth. Season with salt and pepper.

Slice steak against the grain. Divide broccoli, mashed potatoes, and steak between plates. Top steak with sauce. Garnish with remaining chives.