Cook fusilli according to package instructions (gluten free pasta works well too)
Melt butter in a large saucepan, when melted add the flour and stir well
Gradually add the milk whilst continuously whisking the mixture
When all the milk has been added, turn the heat down to the lowest
In a small frying pan sauté the onion and mushrooms in the oil
Add the mascarpone and cheddar cheese to the sauce and stir until melted and well combined
Add the spinach, mushrooms and onion to the sauce
Drain the pasta and stir it into the sauce until completely covered
Serve and Enjoy