Slice mushrooms thickly. Don't crowd them when cooking — cook in batches if needed.
Melt butter in a large pot over medium-high heat. Add mushrooms. Don't stir for 3 minutes — wait for browning. Stir, cook 3 more minutes. Brown mushrooms = deep flavor. Remove ¼ of the mushrooms and set aside for texture.
Add diced onion and garlic to the pot. Cook 5 minutes until soft.
Sprinkle flour over the mushrooms and stir to coat. Cook 1.5 minutes to remove raw flour taste.
Pour in beef broth slowly while stirring to prevent lumps. Bring to a simmer.
Cook 15 minutes. Add heavy cream and stir. Simmer 5 more minutes.
Blend with an immersion blender until smooth, then stir the reserved mushroom pieces back in for texture.
Season with salt and a small splash of soy sauce for extra depth.