Preheat the oven to 180°C, gas mark 4.
Butter a 23cm springform cake tin.
Cut the cauliflower into florets and place in a roasting tin. Drizzle with the olive oil, scatter with sage and roast for 50 minutes or until soft and lightly browned.
Meanwhile, melt the butter in a medium-sized saucepan and sauté the garlic over a low heat for 2 minutes or until softened. Add the flour and cook for 1 minute. Pour in the milk and whisk until thickened. Simmer for 5 minutes. Add the cheese and mustard. Allow to cool.
Chop the cauliflower in 2cm pieces and add to the cheese sauce along with the eggs and cooked pasta.
In batches, wilt the spinach by evenly spreading the leaves out in a microwaveable dish. Place in the microwave on high for 30 seconds and set aside.
Line the prepared cake tin with the spinach leaves then pour the cauliflower mixture into the tin, placing another layer of spinach on top. Bake for 45 minutes until a skewer inserted into the centre comes out clean.
Allow to cool for 10 minutes then unmould. Serve warm.