Preheat the oven to 400°F.
Cook the macaroni according to the packet instructions. Drain well in a colander and set aside.
In the meantime, prep all your ingredients.
Bring a large pan of salted water to a boil and add the cauliflower, cooking until tender. Drain and set aside to cool off completely.
Melt the butter in a large, non-stick pan and add the shallots and garlic. Cook over moderate heat until softened and just starting to caramelize.
Add the cream, sour cream, mustard, and Parmesan to the pan and stir well, allowing the mixture to warm through. Season with salt and ground white pepper.
Remove from the heat and add in all cooked macaroni and the drained cauliflower.
Mix well to evenly coat, then pour the whole lot into a suitable-sized baking dish (like the one you would use for a lasagne).
Sprinkle on some grated cheddar and put it in the oven, baking for 15-18 minutes until the cheese bubbles and starts to brown.