Cook the pasta according to packet instructions. Meanwhile melt the butter over medium heat and cook the mushrooms and onion until soft, about 10 minutes. Add the garlic and rosemary then stir through the flour and cook for another couple of minutes.
Pour in the stock and add the marmite and mustard and cook for another few minutes until thickened then add the pasta and sour cream and stir to coat.
Serve topped with grated parmesan and chopped parsley.
Nutrition information is an estimate and will vary depending on the ingredients used.