Slice the button mushrooms and onion into thin pieces.

Melt butter in a pan and sauté the onion until translucent. Add the mushrooms and cook for a few minutes.

Add flour to the pan and stir gently for 3 minutes to avoid burning. Then pour in the heavy cream and milk, and bring to a gentle simmer.

Blend the mixture using a hand blender or mixer to your desired consistency. Leave small chunks for texture if preferred.

Return the blended soup to the pot, bring to a boil, and season with salt and pepper to taste.