This looks like French onion soup but has the rich mushroom flavour instead of onion. It is...
White Button Mushrooms
White mushrooms are the most versatile. They are tasty eaten raw in salads and with dips. Because their flavour intensifies when cooked, they are also ideal for soups and sauces, casseroles, game dishes, and BBQ skewers.
How to purchase
Look for a round symmetrical cap that fits closely to the stem. The mushrooms should be white or off-white in colour, and firm to the touch. Mature Whites have open veils, darkened caps, and a richer taste when cooked.