Arrange oven racks in top and bottom thirds of oven. Preheat to 450F (230C). Cut tomatoes and onion into wedges. Slice sausages into quarters. Discard loose skins from head of garlic. Then slice off top, exposing cloves but leaving root end intact. Tumble vegetables, sausage and garlic onto 2 large rimmed baking sheets. Drizzle with oil and sprinkle with dried seasonings. Stir. Roast in top and bottom of oven until vegetables are lightly charred, 40 to 50 min. Stir and rotate sheets halfway through roasting.
When roasted, place sausages on a cutting board and tomatoes and onions in a food processor. You may have to do this in 2 batches. Squeeze in garlic and discard skins. Pour a little broth onto baking sheets. Using a wooden spoon, scrape up and stir in any brown bits from baking sheet. Add to food processor. Whirl until fairly smooth. Pour into a saucepan along with remaining broth. Bring just to a boil. Slice sausages and scatter overtop of soup along with fresh thyme. It will keep well, covered and refrigerated, up to 2 days.
Nutrition (per serving)
Roasting the ingredients for this Italian-inspired dish creates maximum flavour with minimal fuss.