Make crispy potatoes: Preheat oven to 425°. Scrub potatoes, dry them, and heap on a rimmed baking sheet. Sprinkle with 1/2 tsp. salt and pour on 1/2 cup oil, then turn to coat well. Spread out in a single layer and roast until crisp, 25 to 30 minutes, stirring every 10 minutes to make sure they're evenly coated with oil.
Remove potatoes from oven. Smash with a potato masher, drizzle with more oil if needed, and season lightly with salt. Roast until browned and really crispy, another 20 to 25 minutes, turning over halfway through. Set aside to cool.
While potatoes are roasting, make feta sauce: In a food processor, blend feta 5 to 10 seconds to break it down. With blade spinning, slowly pour in just enough buttermilk to make sauce as thick and smooth as crème fraîche. If there are still lumps, scrape sides and bottom of processor with a rubber spatula and blend just until smooth. Blend in sumac, with salt and pepper to taste.
Assemble salad: In a large bowl, toss tomato pieces, red onion, and crispy potatoes (and any oil in pan) with vinegar, then season with salt and pepper to taste. Spread feta sauce over a large serving platter and arrange tomato and potato mixture on top. Shingle in herbs and finish with a drizzle of oil.