Heat olive oil in a saucepan over medium heat. Add the chopped onion and cook until soft and translucent, stirring regularly. Add garlic and cook briefly until fragrant, ensuring it does not brown, as burnt garlic turns bitter.

Add sliced mushrooms to the pan and stir well. Cook until they release their moisture and begin to shrink. Allow most of the liquid to evaporate. This step intensifies mushroom flavour without needing additional fat.

Pour in the vegetable stock and add salt. Bring the soup to a gentle boil, then reduce the heat. Let it simmer until the mushrooms are completely soft, and the flavours meld together naturally.

Switch off the heat and blend the soup using a hand blender until smooth. Keep the blender fully submerged to avoid splashing and ensure an even texture without air bubbles.

Return the soup to low heat and stir in low-fat milk and black pepper. Heat gently without boiling. Finish with chopped parsley and turn off the heat once warmed through.