Preheat the oven to 200°C/Fan 180°C Fan/Gas 6.
Cook the macaroni in boiling salted water for 2 minutes less than the instructions on the packet, then drain under cold water to stop the pasta cooking further. Whizz the sourdough and almonds in a food processor until a smallish crumb forms, then set aside.
Heat the butter in a large saucepan over a medium heat and cook the onion and garlic for 2 minutes until soft, stirring. Add the flour and cook, stirring, for a minute until a paste forms. Gradually add 600ml of the milk, using a whisk to create a smooth white sauce.
Turn the heat down to low and let simmer for 3–5 minutes until the sauce has thickened slightly, then season with salt and cracked black pepper. Off the heat, stir in the Cheddar, Dijon mustard, thyme, 50g of the vegetarian grating cheese and the peas.
Whizz the remaining 100ml milk in a blender with the spinach, parsley and basil and add this mixture to the white sauce. Fold in the macaroni and mix well to combine. Pour the green mac ’n’ cheese into a 24 x 18cm baking dish and top with the almond crumb and remaining vegetarian grating cheese.
Bake in the oven for 25 minutes until the crumb topping is golden and the mac ’n’ cheese is bubbling. Leave to rest for 10 minutes before serving. Serve with a few crisp salad leaves simply dressed with good oil and a little lemon juice.