Put the cooked squash and red onion in a pan on a medium-high heat with half the butter, and mash the squash with the back of a fork.
Add a tablespoon of plain flour and mix in. Add the ale, little by little, stirring with each splash, until you have a thick, creamy gravy. Simmer for a few minutes, season well, then add the remaining butter, the mustard and parsley.