Heat oven to 200C/180C fan/gas 6.
Put a large pan of water on to boil and
salt it well. Boil the potatoes until tender,
about 15-20 mins. Drain, return the
potatoes to the pan and place over a low
heat. Add the butter and milk, then mash
until smooth. Remove from the heat and
season with salt. Keep warm.
Meanwhile, brush the mushrooms
generously all over with oil. Place gill-side
up in a baking dish, bake for 15 mins,
then remove from the oven. To make
the rarebit topping, melt the butter in a
saucepan over a medium heat and fry
the onion for 10-15 mins until soft and
translucent. Add the flour and cook
for 1 min. Reduce the heat, then add
the Guinness, cheese, mustard and
seasoning, stirring until the cheese has
melted. Add the eggs and stir constantly
until the mixture thickens, about 5 mins,
but don’t overcook or you’ll end up with
scrambled eggs. Spoon the rarebit mix
into the mushroom caps and set aside.
To make the gravy, heat the oil in a pan
over a medium heat and add the onion
and thyme. Fry for 10-15 mins until the
onion is soft and translucent. Add the
flour and cook, stirring, for 2 mins. Add the
wine, stock and Marmite, then stir until
thick. Simmer for about 5 mins.
Just before serving, heat the grill to
high. Place the mushrooms under the grill
and cook for a few mins until the rarebit
is puffed and golden. Reheat the mash if
you need to, place a spoonful on each
plate along with a mushroom. Spoon
gravy around and over the mash, scatter
with thyme and serve.