Slice the mushrooms into 1cm pieces. Trim the root and top 3cm off the leek. Rinse the leek under cold water to remove any dirt, then shake dry. Halve lengthways and thinly slice. Peel and crush or finely grate the garlic. Peel the potatoes and dice into small chunks. Pick the thyme leaves from the stalks.
Pour 1 tbsp oil into a pan and bring to a mediumhigh heat. Slide in the mushrooms and leeks and sprinkle in a generous pinch of salt and pepper. Fry for 6-8 mins, stirring often, till softened and golden. Add the garlic and most of the thyme leaves (pop a pinch aside for later) and continue to cook for a further 1 min.
Pour 750ml boiling water into the pan and crumble in the stock cube. Slide in the chopped potatoes. Bring to the boil, then reduce to a simmer and cook gently for 20 mins, till the potatoes are very tender.
Use a stick blender to whizz the soup a little, leaving some chunks of mushroom and potato for texture. No stick blender? No problem, simply pour the soup into a blender and whizz.
Ladle the soup into a couple of deep bowls. Sprinkle over the remaining thyme leaves and a crack of pepper to serve.