I had fish finger coupled with potatoes, served with a sauce of mustard today.
Heat butter in a jar, until it is liquid. Then one tablespoon of flour (the white one, not the grey one ). Then you need half a glas of water to mix it with until it reaches the consistency you like. Should not be too solid or too liquid. Then 1 1/2 tablespoon of mustard, a little shot sweetener and a little shoot vinegar. Then you can take something to make it a little bit more flavoursome, like maggi. But not to much or the sauce is ruined xD.
I just love that, can go thousand miles for it xD.
Another idea for this kind of sauce is called "Königsberger Klopse" ).
You take some meat, like used in hamburges. Half cow, half beef. That you can flavour with a little grain of salt and pepper. Then you form little balls out of the meet and cook it for some time. When its done, the butter and the flour is given in the cooked water and a little mustard in the end. That sauce is not based on mustard but on the flavor of the meat, so not to much mustard. In the end you can give one smal glass of caper in the sauce. They are really great in that combination.
Served with potatoes, noodles, just how you like it. I prefer potatoes .