Halve the potatoes (quarter any large ones). Slice carrots into 3 cm / 1.2" diagonal triangles, alternating angle with each cut. Cut the parsnip into 4 cm / 1.5" lengths, then into even batons. Half the red onion and slice into 4 wedges, then cut each wedge into 2–3 pieces (~2 cm / 0.8" squares). Smash each garlic clove by pressing with the flat side of a knife until it bursts but stays mostly intact.

Place the red onion pieces in one bowl and toss with 1 tbsp olive oil and a small pinch of salt and pepper. Set aside so the onion can be added later and not overcook.

In a larger bowl combine potatoes, carrots, parsnips, smashed garlic, and thyme sprigs. Hand-tear sage leaves and stems into the bowl. Drizzle with the remaining 3 tbsp olive oil plus the rest of the salt and pepper, then toss until everything is well coated.

Preheat oven to 200°C / 390°F (180°C fan-forced). Spread seasoned root vegetables in a single layer on a large baking tray — do not crowd the pan. Roast undisturbed for 30 minutes until edges begin to turn golden.

After 30 minutes, scatter the seasoned red onion over the tray. Give everything a quick toss with a spatula to redistribute caramelized bits from the bottom of the tray.

Return the tray to the oven and roast for a further 60 minutes, tossing once halfway through at the 30-minute mark. Total roasting time is 1 hour and 30 minutes for slow, even caramelization — do not crank the heat.

Remove the tray. Squeeze the roasted garlic from its skins so the sweet, caramelized garlic mixes through the vegetables. Sprinkle with chopped parsley and serve straight from the tray.