Preheat the oven to 340°F.
Make the Soup: Cut the cauliflower into florets and place in a large roasting tray. Drizzle with the olive oil and season with salt and pepper. Cut the garlic bulb in half around its circumference, wrap it tightly in foil and place it in the corner of the same roasting tray. Place the tray in the oven and bake until the cauliflower has some color, 18 to 20 minutes. Remove the cauliflower from the tray and place the garlic back in the oven for 10 minutes to finish cooking.
In a large saucepan over medium heat, fry the leek in the butter until it’s softened, about 5 minutes. Add the curry powder and cook for about 2 more minutes.
Place the roasted cauliflower in the saucepan and squeeze the garlic cloves from the bulb into the pan. Add the chicken stock and bring to a simmer. Cover and cook over medium heat for 10 minutes.
Make the Chili-Garlic Oil: Meanwhile, add the minced garlic, chili flakes and parsley to a heatproof bowl. Heat the oil in a saucepan until very hot, then pour this over the garlic mixture. It will sizzle vigorously for a bit. Mix and cool before serving.
Remove the soup from the heat and blend it using an immersion or countertop blender. Taste and adjust the seasoning as needed.
Serve in soup bowls with a drizzle of the chili oil and toasted sourdough croutons.
