Preheat the grill to high. Melt the butter with 1 tablespoon of oil in a large saucepan over a medium heat. Add the leeks, carrots, thyme and a pinch of salt. Cook for 8-10 minutes, stirring occasionally, until softened.
Add the garlic and cumin and cook for 2 minutes, then add the cauliflower and cook for another 2 minutes. Pour over the hot stock, cover and bring to the boil. Remove the lid and simmer for 5 minutes.
Meanwhile, cut the baguette at an angle into 12 thin slices, drizzle with the remaining oil and sprinkle with salt. Grill for 1-2 minutes until golden; turn over and crumble the feta on top. Grill for 2-3 minutes more, until golden. Remove and garnish with thyme leaves.
Blitz the soup until very smooth and season. Serve in warmed bowls with a swirl of soured cream and the bruschettas on the side.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.