In a frying pan, heat 1 tbsp of olive oil. Add the diced onion and sliced garlic, cooking on medium heat for 5 minutes or until softened. Add the sliced mushrooms with 1 tsp of butter, increase the heat to medium-high, and cook for another 5 minutes. Once the mushrooms are ready, remove from the heat and set aside.
In a large pot, add the vegetable stock, diced carrots, celery, leek, potato, and bay leaf. Bring to a boil, then reduce to a simmer. Cook for 10 minutes.
Add the uncooked penne pasta and cook for another 10 minutes.
Stir in the fried mushrooms, onions, and garlic, cooking for an additional 5 minutes.
By now, the pasta should be cooked through, and the mushrooms should still hold their shape. If the soup is too thick, add a little water.
Remove one ladle of soup liquid (without solids) and mix it with the double cream. Pour the mixture back into the pot to prevent the cream from splitting.
Adjust the seasoning to taste and serve, garnished with freshly cracked black pepper and chopped parsley. Optionally, stir in 1 tsp of yoghurt per plate for added sourness.