Put the onions, garlic, cauliflower, milk, stock, paprika and some seasoning in a large pan with a lid. Bring to the boil and cook for 12 minutes or until the vegetables are soft then allow to cool slightly and blitz with a stick blender until smooth.
Add most of the grated cheese and the spinach to the pan, putting the lid back on to allow the cheese to melt and the spinach to wilt.
Meanwhile, cook the macaroni according to the instructions, then drain.
Preheat the grill to high.
Mix the drained pasta with the cauliflower cheese sauce and transfer to an oven-proof dish, then scatter over the remaining grated cheese and the breadcrumbs. Cook under the grill for 3-4 minutes, until golden and bubbling.