1. Adjust oven rack to lowest position, place rimmed baking sheet on rack, and heat oven to 500°F.

2. Bring 10 cups water to boil in Dutch oven over high heat. Add potatoes and baking soda. Return to boil and cook for 1 minute. Drain potatoes. Return potatoes to Dutch oven and place over low heat. Cook, shaking pot occasionally, until any surface moisture has evaporated, about 2 minutes. Remove from heat. Add butter, 1 1/2 teaspoons salt, and cayenne; mix with rubber spatula until potatoes are coated with thick, starchy paste, about 30 seconds.

3. Remove baking sheet from oven and drizzle with 2 tablespoons oil. Transfer potatoes to baking sheet and spread into even layer. Roast for 15 minutes. While potatoes roast, combine onions, remaining 1 tablespoon oil, and 1/2 teaspoon salt in bowl.

4. Remove baking sheet from oven. Using thin, sharp metal spatula, scrape and turn potatoes. Clear about 8 x 5-inch space in center of baking sheet and add onion mixture. Roast for 15 minutes.

5. Scrape and turn again, mixing onions into potatoes. Continue to roast until potatoes are well browned and onions are softened and beginning to brown, 5 to 10 minutes. Stir in chives and season with salt and pepper to taste. Serve immediately.