Heat oil in large saucepan; add onion, celery, leek, garlic, thyme, marjoram and bay leaf.
Cook until vegetables are tender.
Stir in potatoes and stock.
Bring mixture to a boil.
Reduce heat to low; simmer until potatoes are tender.
Remove bay leaf.
Place soup in blender; blend until smooth.
Return to pan.
Heat 2 tablespoons butter in small skillet; add mushrooms.
Cook until tender; add to soup.
Stir in cream and remaining butter.
Season to taste with salt and pepper.
Sprinkle each serving with chives.