1. Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill a large pan with water, add a pinch of salt and bring to the boil.
2. Peel the onion and thinly slice it. Peel and crush or grate the garlic. Grate the cheddar. Drain the mozzarella and tear into small pieces. Scoop both types of cheese into a bowl.
3. Pour 1 tbsp olive oil into a pan and warm on a medium heat. Add the onion and garlic, season with a pinch of salt and pepper and fry for 5 mins, stirring occasionally, till softened.
4. While the onion is cooking, trim the base off the cauliflower. Break off the leaves and set them to one side. Cut the cauliflower into 6 large wedges.
5. The water in the pan should be boiling. Tip in the pasta and the cauliflower wedges and leaves. Simmer for 8 mins, then drain.
6. Sprinkle the flour into the onion pan and stir well. Continue to fry for 2 mins. Whisk in 1 tbsp mustard and a quarter of the milk, scraping the bottom of the pan, till the onions thicken into a paste. Slowly pour in the rest of the milk, continuously whisking, till combined. Bring to a simmer, then stir in the cheddar and mozzarella.
7. Tip the pasta and cauliflower into a large ovenproof pan or a casserole dish. Pour over the cheese sauce and give everything a stir to coat. Season with a pinch of black pepper. Slide into the oven and bake for 20 mins, till golden brown at the edges.
8. While the cauliflower mac and cheese is baking, trim the kale leaves offany thick cores, and roughly shred the leaves. Pour 1 tbsp olive oil into a frying pan and warm to a medium-high heat. Add the kale, along with 3 tbsp water and a little salt and pepper. Stir fry for 4-5 mins, till tender. Squeeze in the juice from the lemon and stir well.
9. Serve the cauliflower mac and cheese with the lemony kale on the side.