Begin by preparing the vegetables. Chop the cauliflower, broccoli, kale, and onion into manageable pieces. Place them on a baking tray, drizzle generously with olive oil, and roast in a preheated oven until lightly coloured and tender when pierced with a fork.

Once roasted, transfer the vegetables into a blender. Add a suitable amount of vegetable stock to assist with blending and achieve the desired consistency, and crumble in the blue cheese.

Blend until smooth. Taste and season with freshly ground pepper as needed. If the soup is too thick, additional stock can be added to thin it to your preference.

Serve the soup warm, ideally garnished with a few extra crumbles of blue cheese or a drizzle of olive oil if desired.