Heat oven to 200C/180C fan/gas 6. Remove the leaves from the cauliflower and trim the stalk end, then cut into chunks. Blitz half the cauliflower in a food processor until finely chopped, like rice. Transfer to a bowl and repeat with the remaining half. Tip all the cauliflower in a bowl, cover with cling film and microwave on High for 5-6 mins until softened. Tip onto a clean tea towel and leave to cool a little. Once cool enough to handle, scrunch up the tea towel and squeeze as much liquid as you can out of the cauliflower, then transfer to a clean bowl. Stir in the ground almonds, egg, oregano and plenty of seasoning. Line a baking tray with baking parchment and grease with oil. Mound the cauliflower mix into the centre of the tray, then use a spoon and your hands to spread out into a 30cm round. Make it a little thicker at the edges to create a ‘crust’. Bake for 15-18 mins until golden brown and starting to crisp a little at the edges.
Meanwhile, heat a griddle pan, brush each aubergine slice on both sides with a little of the oil, season and cook for 5-6 mins, turning once, until softened and charred – you’ll need to do this in batches. Transfer to a plate. Brush the onions with oil, season and griddle for 5-8 mins until softened and charred. To make the tomato sauce, whizz the canned tomatoes, tomato purée, garlic and some seasoning in a blender until smooth. Transfer to a small saucepan, bring to a simmer and cook gently for 8-10 mins until thick (you don’t want any watery tomato soaking into the cauliflower base). Tear half the basil leaves and stir through the sauce.
Once the cauliflower base is cooked, set aside to cool a little. Turn the oven up to 240C/220C fan/gas 8. Drain the mozzarella and pat dry with kitchen paper. Spread the tomato sauce over the base, sprinkle over the Grana Padano, then arrange the aubergines, red onion and mozzarella on top. Scatter over the chilli flakes and return to the oven for 10-12 mins. Before serving, shave over a little more cheese and scatter with the remaining basil leaves.