Heat oven to 200C/180C fan/gas 6.
Put the onions in a roasting tin with the
oil, salt and pepper. Give it a good stir,
then roast for 45 mins, stirring halfway
through, until the onions are tinged brown,
but not burnt.
Tip the onions into a large pan with the
stock, mustard and Marmite. Bring to
the boil and simmer for 15 mins, then stir
in the parsley. Toast 4 of the bread slices
then scatter on the cheese. Ladle the
soup into bowls, pop a toast into each
and serve with the extra slices of bread
on the side.