1½ lb. tiny potatoes (such as creamers or peewees)
2 Tbsp. extra-virgin olive oil, plus more for onions
¾ tsp. garlic powder
¼ tsp. sumac
1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, plus more
Freshly ground black pepper
2 small or 1 medium red onions, sliced into ½”-thick rounds, layers separated
⅓ cup sour cream
¼ cup (packed) coarsely chopped mixed tender herbs (such as parsley, dill, and/or chives)
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.
Meanwhile, place onions in a medium bowl. Drizzle a little oil over and season with salt and pepper; toss to coat.
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.
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.