Preparation time: 15 min.
Remove the mushroom caps from the stems. Slice the caps and finely chop the stems.
Heat the oil and half the butter in a heavy pan and add onion, mushroom stems and about ¾ of the caps.
Fry for 1-2 minutes, stirring frequently, cover the pan and simmer everything on a low heat for 6-7 minutes, stirring occasionally.
Stir in the flour and cook for about 1 more min. Gradually add stock and milk until a smooth sauce forms.
Add basil and season to taste.
Bring to boil, partially cover the pan and simmer gently for 15 min.
Let the soup cool a little, then pour it into the blender and puree it smooth.
Melt the rest of the butter in a frying pan and gently fry the remaining mushroom caps for 3-4 minutes until they start to soften.
Pour the soup into a clean pan and stir in the fried mushrooms.
Add cream if necessary and sprinkle with basil leaves.