Add potatoes, carrots, parsnips and onions to a large bowl. Drizzle evenly with the olive oil, sprinkle with a few generous pinches of salt and pepper, then toss gently to coat all of the vegetables (being careful not to break up the onion chunks, if possible).
Spread the vegetables out evenly on a baking sheet. Sprinkle evenly with a little extra salt and pepper.
Bake for 20-25 minutes, or until the vegetables are tender and cooked through, flipping once halfway through cooking.