This one can be a sauce if you don't add stock- Slowly saute' sweet onions in butter and a little light olive oil with frequent sirring until they have turned an even gold, then add garlic & thyme. As soon as garlic is a bit transparent, add thickly sliced portobello mushrooms or whatever fungus you like along with some lemon juice, saute a bit longer. Add some dry white wine and possibly Worcestershire and/or balsamic vinegar, salt, black pepper and a tiny pinch of sugar, then stir in a bit of medium roux when the mushrooms have released liquid. Taste. If you overdid either the lemon & vinegar or the sugar, adjust by adding more of one or the other to offset the excessive sweetness or sourness. If you're making it as a side to a nice steak, stop there. For soup, add your stock of choice, perhaps some additional herbs and more roux as liked.