Heat a large saucepan up and add oil, garlic, leek and shallot. Fry off until golden. Add cauliflower florets and and stir. Add enough water to cover cauliflower. Add in stock cubes and cashews and bring to the boil. Boil for around 15 – 20 minutes until cauliflower has softened. Let it cool down a bit before transferring to the blender.
Transfer to a blender and blend until creamy, season with salt and pepper.
Preheat the oven to 180°C.
Lightly oil a large cast iron skillet or shallow casserole dish with olive oil. Pour in white sauce to fill half way. Arrange the sliced potatoes and zucchini in circles.
Sprinkle the mozzarella over the top, then cover the baking dish.
Bake covered for 60 minutes or until the potatoes are tender when pierced with a knife. Remove the cover and bake an additional 10 minutes to brown the top. If needed move the dish to under the broiler at the end for additional browning. Garnish with fresh herbs and serve warm.