Step 1

Preheat oven to 450°. Place potatoes on a rimmed baking sheet. Drizzle 2 Tbsp. oil over, sprinkle with garlic powder, sumac, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, and season with pepper. Toss to coat, then arrange potatoes in an even layer. Roast until nearly tender and beginning to brown in spots, 18–22 minutes.

Step 2

Meanwhile, place onions in a medium bowl. Drizzle a little oil over and season with salt and pepper; toss to coat.

Step 3

Carefully remove potatoes from oven and shake baking sheet to turn potatoes and scoot to one side. Arrange onions in a single layer in empty space on baking sheet; reserve bowl. Return baking sheet to oven and roast potatoes and onions until potatoes are browned and fork-tender and onions are charred around the edges, 13–17 minutes. Let cool slightly.

Step 4

Scoop sour cream into reserved bowl; season with salt and pepper. Spread sour cream onto a platter or into a large shallow bowl and top with potatoes and onions. Scatters herbs over and sprinkle generously with pepper.