Preheat the oven to 200 degrees Celsius. Grease two large baking dishes with olive oil.

Add the half of the diced pumpkin, halved tomatoes, celery pieces and quartered brown onions to each dish. Drizzle with olive oil and season with salt and pepper. Toss to coat evenly and then spread all the vegetables out evenly. Nestle the halved garlic heads into the vegetables and then place the baking dishes into the oven. Bake for one hour. At about the halfway point, carefully move the bottom dish to the middle shelf and the middle shelf dish to the bottom shelf to help with even cooking. After an hour, remove from the oven and set aside. Reduce the oven temperature to 180 degrees Celsius.

Meanwhile, make the croutons. Add the torn-up bread pieces to a medium bowl along with the olive oil, rosemary and sea salt flakes. Toss to coat all the bread evenly. Tip the bread mixture out onto a baking tray and spread out evenly. Place into the oven to bake for 10 minutes or until golden and crispy. Remove from the oven and set aside.

Remove the heads of garlic from the roasted vegetables. Tip the vegetables into a large casserole pot and squeeze the garlic out of the skins and into the pot. Add the chicken stock and chopped basil leaves and then bring to a boil. Once boiling, reduce to a simmer for 5 minutes.

Turn off the heat. Carefully use an immersion blender to blend the soup until smooth and creamy. Serve topped with the crispy croutons, some basil and a drizzle of olive oil.