For the cheese sauce:
In a pan add, butter and plain flour and cook for 2-3 minutes.
Then slowly add the stout and stir continuously.
Then add the milk slowly until fully combined, fold in grated cheddar and whole grain mustard.
For the crispy onion crumb:
Combine all ingredients and lightly toast in the oven at 170ºc for 10-15 mins, stirring regularly.
Fold the cold pasta into the hot sauce, top with the onion crumb and garnish with chives.