Slice up the leek, reserving only the white and light green parts (not the dark green leaves or the roots if still attached). Add to a small bowl of water and agitate with your fingers to loosen any dirt. Drain and rinse. In a large pot, heat the olive oil over medium heat and add the leek. Cook for 3-4 minutes.

In the meantime, give the garlic a rough chop as well as the cauliflower and potato (once peeled). Add these ingredients to the pot also and season generously with salt and pepper as well as the cayenne. Cook another 2 minutes. Then add the vegetable stock (or chicken stock if using), reduce the heat to low, cover, and allow to cook until the potatoes are soft enough to mash (about 15-18 minutes).

Use an immersion blender to puree all the ingredients. You could also do this in a good blender; just be careful as you transfer the hot liquid back and forth. Once the soup is blended well, turn off the heat and stir in the milk or cream. Taste and add more salt and pepper, if needed.