How do I get rid the yellow layer of oil that forms in my cream of mushroom soup? When making cream of mushroom, I sauté the chopped up wild mushrooms with unsalted butter until tender and add a little olive oil. After a while, I add the milk, cream, bay leaves and season it to taste. And it tastes great. ;-)

However, there's always this layer of oil that forms which gives the soup a yellowish tinge. I usually skim off that layer of oil with a spoon and all is well. 

Is there a way of preventing it from forming in the first place? Am I doing something wrong?
Yes, cream soups are typically roux-based. If you add some flour (roughly an equal amount to the butter) to the fats, whisk and cook the flour for a couple minutes and then slowly whisk in the milk, it will be much less likely to separate. The soup will also be thicker, which I would imagine is a good thing.