Peel and finely slice the onions.
Place a large lidded pan on a medium heat, add two tablespoons of olive oil and the chopped onions.
Crumble a stock cube over the onions, season with salt and pepper, then strip thyme leaves and add them to the pan.
Squash two cloves of garlic through a garlic crusher and add them to the pan.
Remove the stalks from portobello mushrooms, tear them into chunks and place them on a griddle pan. Cook for a few minutes until charred.
Place the portobello mushroom stalks into the pan with the onions, then boil a kettle.
Cook the ingredients in the onion pan for a few minutes then pour in the boiling water.
Use a stick blender to puree the soup until thick.
Add the single cream and truffle oil to the blended soup.
To make the toasted sandwich, destalk the chestnut mushrooms and place them on the griddle pan, then cook until charred.
Add four slices of ciabatta bread to the griddle pan, grilling both sides and rubbing with a halved garlic clove.
Chop an apple into thin matchstick slices and roughly chop the parsley leaves. Pour lemon juice over both.
Place the chestnut mushrooms on the toast, crumble the stilton cheese and walnuts on top, and place it in the oven until the cheese melts.
Serve the toast with the apple and parsley on the side.
