Preheat the oven to 350°F. Trim, halve and wash the leek and peel the garlic, then finely slice with the broccolini stalks, reserving the florets for later. Place the sliced
veg in a large casserole pan over a medium heat with the butter, then strip in the thyme leaves and cook for 15 minutes, or until softened, stirring regularly. Stir in the flour, followed slowly by the milk, then simmer for 10 minutes, or until thickened, stirring regularly. Meanwhile, cook the pasta in a large pan of boiling salted water for 5 minutes, then drain.
Grate the Parmesan and most of the cheddar into the sauce, and mix well. Tip into a blender, add the spinach and whiz until smooth—you may need to work in batches. Season to perfection with sea salt and black pepper, then stir through the pasta and broccolini florets, loosening with a splash of milk, if needed. Transfer to a 14x10-inch baking dish, grate over the remaining cheddar and scatter over the almonds.
Bake for 30 minutes, or until beautifully golden and bubbling.