Preheat your oven to 400°F (200°C) and lightly grease a large rimmed baking sheet.
Toss the onion wedges in a large bowl with the olive oil, thyme, garlic powder, salt, and pepper until fully coated.
Spread the onions across the baking sheet in a single layer to ensure they caramelize rather than steam.
Nestle the sausages into the onions, making sure they have direct contact with the pan for a perfectly snapped skin.
Roast for 25-30 minutes, tossing the onions halfway through, until they are soft, golden, and translucent.
Drizzle with balsamic vinegar during the last 5 minutes of roasting to add a tangy, sweet glaze to the dish.
Increase the heat to a broil for 1-2 minutes if you want extra char on the sausage tops.
Serve hot directly from the pan for a rustic, soul-warming meal that celebrates simple flavors.