Preheat the oven to 200 degrees fan forced.
Put a large frying pan on a medium heat and add the olive oil and 1 tablespoon butter, let it get hot and melt. Add the leek, celery and sauté for 3-4 mins. Then add the chopped cauliflower. Sauté for 10 mins. If the bottom of the pan starts to brown or burn, just add a tablespoon of water each time to the brown spot to stop it from burning.
Add the flour and stir for 2 minutes (to make sure it cooks out). Add 2 tablespoons butter and combine. Follow by adding the mustard, milk and stock/water and stir for about 5 mins. Season well with sea salt & cracked black pepper. Then stir through the parmesan, sour cream and tasty OR mozzarella cheese.
Meanwhile, place a large saucepan of salted water onto boil. Cook the pasta for approx. 8 minutes, it will still be a little hard, but will continue cooking in the oven. Reserve a cup of starchy cooking water, drain the pasta and pour into a large baking dish (approx. 24cm X 20cm).
Pour the creamy sauce over the pasta and mix through really well. You might want to do this in stages so you really get the sauce through the pasta. If it seems a bit dry, then add a little of your starchy pasta water here until you reach a good consistency.
For the topping, mix all ingredients together and sprinkle over the bake. Season well with sea salt & black pepper.
Cover with foil and bake for 25 minutes. Remove the foil and turn the oven up to 220 degrees fan forced and bake for a further 10 mins. Serve while hot.