Preheat the oven to 425 degrees F.
Spread the potatoes, sweet potatoes, carrots, parsnips, and red onions out on a heavy, rimmed baking sheet. Drizzle with the olive oil, salt, and pepper, and toss to coat evenly. Roast in the oven for 50 minutes, flipping with a spatula half way through the cooking. Transfer to a serving bowl, drizzle with butter, and sprinkle with parsley and flaky sea salt.