Wash the mushrooms under running lukewarm water. Do not defrost - we will be cooking frozen washed mushrooms.
Place mushrooms into a medium soup pot and cover with 1.5l of cold water.
Add 1 bay leaf, 1/2 tsp of black peppercorns and bring to boil.
Simmer for 20-25 minutes or until mushrooms are cooked.
In the meantime, heat 2 tbsp of olive oil in a medium frying pan.
Add the onions and fry for 4-5 minutes until just starting to turn golden.
Add the grated carrot and fry for further 3-4 minutes until it softens. Season with 1/2 tsp salt and set aside.
When the mushrooms are cooked, take them out and transfer to a plate.
Add 1/2 tsp of salt to the pot, add the potatoes and simmer for 7-10 minutes until the potatoes are done.
Return the mushrooms and the onions and carrots to the pot and stir to combine. Taste and adjust for salt.
Turn the heat off and stir in the spinach leaves. Keep stirring until they just wilt.
Sprinkle over 1/3 tsp of grated nutmeg.
Serve garnished with fresh chopped parsley and sour cream.