Preheat oven to 425°F (220°C). Line a rimmed sheet pan with parchment paper.
Prep vegetables: cut potatoes into chunks, slice carrots thick, and cut parsnip into chunks similar in size. Smash garlic cloves. Cut onion into wedges.
In a large bowl, toss potatoes, carrots, parsnip, and garlic with olive oil, salt, black pepper, and dried thyme.
Spread vegetables on the sheet pan in a single layer. Roast for 20 minutes.
Add onion wedges, toss everything, then spread back into an even layer.
Roast 15–20 minutes more, until potatoes are fork-tender and edges are browned.
Finish with parsley and fresh basil, then serve hot.