Cut potatoes in half. (For large, cut into 3 or 4)
Cut carrots into 3cm / 1.2" triangles on the diagonal.
Cut parsnip into 4cm / 1.5" lengths, then cut into batons/chunks.
Cut red onion in half, then cut each half into 4 wedges, then into 2cm/0.8" squares.
Smash garlic cloves with the side of a knife.
Preheat oven to 200°C / 390°F (180°C fan).
Toss onion in one bowl with 1 tablespoon oil, and a pinch of salt and pepper. Set aside.
Put all other vegetables, thyme, and garlic in a separate bowl. Tear sage leaves and add to the bowl. Toss with remaining oil, salt, and pepper. Spread on a large baking tray.
Roast for 30 minutes. Add onion, toss vegetables, and roast for a further 1 hour, tossing halfway, until cooked through with golden edges.
Sprinkle with parsley and serve immediately.
