Wash and dry all produce. Peel and dice potato into 1/2-inch cubes. Place in a medium pot with enough salted water to cover by 1 inch. Cook until tender when pierced with a fork, 15 minutes. Reserve 1/4 cup potato cooking water, then drain and return to pot. Cover.

Meanwhile, trim and thinly slice scallions, separating whites from greens. Mince or grate garlic. Trim and thinly slice mushrooms.

Pat steak dry with paper towels; season with salt and pepper. Heat a large drizzle of oil in a large pan over medium-high heat. Add steak and cook to desired doneness, 4-7 minutes per side. Remove from pan and set aside to rest.

Add sour cream, cheese, and 1 TBSP butter to pot with potato. Using a potato masher or fork, mash over medium-low heat until smooth. (TIP: If needed, stir in reserved potato cooking water, a little at a time, until mixture is smooth.) Season generously with salt and pepper.

Heat a drizzle of olive oil in pan used to cook steak over medium-high heat. Add mushrooms and cook, stirring, until lightly browned and softened, 2-4 minutes. Add scallion whites and garlic and cook until fragrant, about 1 minute. Stir in stock concentrate and 1/3 cup water, then bring to a boil. Once boiling, reduce heat to low. Simmer until slightly thickened, 1-2 minutes. Turn off heat and stir in 1 TBSP butter. Season with salt and pepper.

Slice steak against the grain. Divide steak and mashed potato between plates. Top steak with sauce. Garnish with remaining scallion greens.