Ok this is very technically not right, but I'm not a chef in a fancy French restaurant, so there's no one to faint at this or fire me on the spot. What I do when I make (western-style, duh.) soup, after I blend the ingredients and before I add the extra liquid to turn it into the desired consistency, I set aside a bit. That thick velvety puree is wonderful as a sauce spooned over meats or fish or vegetables that are simply grilled/poached/steamed and you get a great sauce consistency without the need for flour or other thickeners!
Creamy Celeriac, Hazelnut and Parsley Sauce
refer to my previous post on Celeriac and Hazelnut Soup ;)
plus chopped fresh parsley
You can try adding some mustard to make a mustard and celeriac sauce, which will go brilliantly with stronger meats or even game.
This velvety sauce was poured over a simple pan-fried salmon steak.