Adjust rack to middle position and preheat oven to 450 degrees. Wash and dry produce.

Slice potatoes into ¼-inch-thick rounds. Halve, peel, and finely chop half the shallot (all for 4 servings).

In a small bowl, whisk together half the vinegar (you’ll use the rest later), 1 TBSP olive oil (2 TBSP for 4), and ½ tsp sugar (1 tsp for 4). 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), a couple big pinches of salt, and pepper; toss until potatoes are completely coated. Arrange on prepared sheet into clusters (about six slightly overlapping slices each).

Roast on middle rack for 12 minutes. Remove from oven; sprinkle with Parmesan.

Return to oven; roast 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, 5-7 minutes per side.

Turn off heat; transfer to a cutting board to rest. Wipe out pan.

Heat a drizzle of oil in same pan over medium heat. Add shallot; cook, stirring, until softened, 1-2 minutes.

Pour in remaining vinegar. Simmer until reduced by half, 30 seconds.

Stir in stock concentrate and ¼ cup water (⅓ 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) until melted and combined. Season with salt and pepper.

In a second large bowl, toss mixed greens with as much vinaigrette as you like. Season with salt and pepper.

Slice steak against the grain.

Divide steak, potato clusters, and salad between plates. Top steak with sauce and serve.