Adjust rack to middle position and preheat oven to 450 degrees. Wash and dry all produce. Slice potatoes into 1/4-inch-thick rounds. Halve and peel shallot; finely chop half (save other half for another use). In a small bowl, whisk together half the vinegar (you’ll use the rest later), 1 TBSP olive oil, and 1/2 tsp sugar (for 4 servings, use 2 TBSP olive oil and 1 tsp sugar). Season with salt and pepper.

Lightly oil a baking sheet. In a large bowl, combine potatoes, garlic powder, 2 TBSP olive oil (4 TBSP for 4 servings), and a couple big pinches of salt and pepper; toss until potatoes are completely coated. Arrange on prepared sheet into clusters (about 6 slightly overlapping slices each). Roast on middle rack for 12 minutes. Remove from oven; sprinkle with Parmesan. Return to middle rack until potatoes are tender and cheese is lightly browned, 10-12 minutes more.

While potatoes roast, pat steak dry with paper towels; season all over 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, 3-6 minutes per side. Turn off heat; transfer to a cutting board to rest. Wipe out pan.

Heat a drizzle of oil in pan used for steak over medium heat. Add chopped shallot; cook, stirring, until softened, 1-2 minutes. Pour in remaining vinegar. Simmer until reduced by half, 30 seconds. Stir in stock concentrate and 1/4 cup water (1/3 cup for 4 servings). Simmer until thickened, 2-3 minutes. Turn off heat. Stir in sour cream and 1 TBSP butter (2 TBSP for 4). Season with salt and pepper.

While sauce cooks, toss mixed greens in a second large bowl with as much vinaigrette as desired. Season with salt and pepper.

Slice steak against the grain. Divide steak, potato clusters, and salad between plates. Top steak with sauce and serve.