Place the frozen cauliflower into a microwave-proof bowl. Trim off the white part of the leek (putting the green top aside for later), add to the bowl and cook in the microwave on high (800W) for 10 minutes, or until the cauliflower is defrosted and the leek has softened. Meanwhile, place a large non-stick frying pan on a medium-high heat with 1 tablespoon of olive oil. Peel, finely slice and stir in 1 clove of garlic and cook for 2 minutes, or until golden, then very finely slice and add the green leek top. Roughly chop the bread to create rustic breadcrumbs, add to the pan and fry for 5 minutes, or until starting to crisp up, stirring occasionally. Finely chop and add the nuts, stirring until toasted, then transfer everything to a plate. Return the pan to the heat with 1 teaspoon of oil. Peel, finely slice and add the remaining garlic and fry for 2 minutes, or until golden. Slice and add the softened white part of the leek, then stir through the dried thyme and softened cauliflower. Stir the flour into the pan, then gradually add the milk, a ladleful at a time, stirring continuously until you have a smooth sauce. Remove from the heat, then either blitz it with a hand blender until silky smooth, or use a potato masher to mash everything together to create a rustic sauce. Grate in most of the cheese, stirring gently to combine. Cook the pasta in a pan of salted boiling water according to the packet instructions, then drag it straight into the sauce, taking a little cooking water with it. Toss together, adding a splash of starchy cooking water to loosen, if needed. Divide between plates, sprinkling over the crispy breadcrumbs and grating over the reserved cheese, to serve.