Place cheeses in mixing bowl; blend for 10 sec/speed 7 or until the consistency of coarsely grated parmesan. Transfer to a small bowl.
Place pumpkin, cauliflower, garlic and milk in mixing bowl; cook for 25 min/100°C/speed 1, without measuring cup inserted into lid, or until vegetables are soft. Return measuring cup to lid. Blend for 15 sec/speed 8 or until sauce is very smooth and silky.
Add pasta to mixing bowl through hole in lid; stir to combine. Position steaming dish over mixing bowl and cover. Cook for 14 min/steam mode/reverse/stir mode, adding broccolini to steaming dish after 8 minutes, or until pasta and broccolini are al dente and sauce thickens.
Remove steaming dish. Add three-quarters of the grated cheeses and nutmeg to pasta mixture; stir to combine. Season to taste. Sprinkle mac 'n' cheese with remaining grated cheeses and extra nutmeg. Serve with broccolini.