Preheat oven to 200˚C, fan on.Place portabellos on a large baking tray, season with salt and pepper.
Roast in the middle of the oven for ±7 minutes until tender and juicy. Allow mushrooms to rest, drain off their liquid and slice into large chunks.
In a large pot of salted, boiling water, cook the macaroni until al dente.Using a jug, scoop out a generous cup of pasta cooking water and set aside.Drain the macaroni.
While the macaroni is cooking, place cream cheese, cheddar, mustard, onion powder, garlic powder and nutmeg in a large saucepan.
Switch heat onto very low and allow everything to melt together. When the cream cheese is almost melted, pour in a little pasta water, mixing well until you achieve a smooth silky sauce.
Add the drained macaroni and the fresh chives, stir to combine. Keep adding a little extra pasta water to keep things saucy.
Finally fold in the large chunks of roasted mushrooms.Taste to adjust seasoning.
Scoop mixture into a baking dish and top with remaining cheddar and breadcrumbs.
Transfer to the oven and bake for 10 - 15 min until the top is golden brown and bubbling.Scoop into bowls, top with some fresh basil and enjoy!