In a pot of boiling salted water, cook pasta al dente. Drain. Meanwhile, heat a little olive oil in another saucepan over medium heat. Cook the chorizo for 3 to 4 minutes. Set aside on a plate.
In the same pan, cook the leeks for 4 to 5 minutes over low-medium heat, stirring occasionally. Set aside on a plate.
In the same saucepan, cook the cauliflower and onion for 4 to 5 minutes, stirring occasionally.
Add the milk mixture and bring to the boil. Simmer over low heat for 5 to 7 minutes until tender. Remove from heat.
Using an immersion blender, emulsify the cauliflower mixture until smooth.
Gradually add the cheese to the pan and heat over low heat, stirring, until melted.
Add macaroni, leeks and chorizo. Season with salt and pepper and stir.
If desired, preheat the oven to broil. Divide the mac n cheese among four cassolettes, top with panko breadcrumbs, and bake for 2 to 3 minutes, until golden brown.