Pour the boiling water into a large bowl and add the porcini mushrooms. Set aside to infuse for 20 minutes.
Meanwhile, heat the oil in a frying pan over a medium heat. Brown the beef cheek on all sides for 3-4 minutes, then transfer to a large lidded casserole.
Add the onions to the frying pan and fry for 4-5 minutes, or until softened and just golden-brown. Transfer to the casserole. Repeat the process with the butternut squash.
Pour the porcini liquid and the mushrooms into the casserole and bring to the boil. Reduce the heat until the mixture is simmering. Half-cover the casserole with the lid, then simmer gently for 3 hours, stirring occasionally and topping up with boiling water if the volume of liquid reduces significantly.
After 3 hours, tip the orzo into the casserole and continue to cook for a further 10 minutes, or until tender.
Meanwhile, slice the top off the sourdough loaf and hollow out the inside.
Ladle the broth into the empty sourdough loaf, sprinkling over the chopped parsley to garnish.