Using yellow mustard as a soup base

You can, I did, but it probably isn't authentic.

The first recipe I found specifically cautioned against yellow mustard. Other recipes call for wholegrain, Dijon, or something rarer. The first recipe also only used 3tbsp to serve 4, which wouldn't use your mustard up very quickly. On the other hand yellow mustard is far less mustardy than Dijon. Wholegrain mustard can be quite vinegary, so that may not be too much of a worry.

The recipe didn't give a reason for avoiding yellow mustard; I suspect it wouldn't be strong enough. I'd try it, with the following changes:

• don't add any salt until the very end
• choose a low salt stock
• use more mustard than called for. I'd start with double, possibly adding more towards the end.

I gave it a try, following the recipe at the 2nd link above (which treats ham/bacon as optional; I opted against).

Having never had the original I can't speak for the authenticity, but mine was quite enjoyable. I got a tasty soup that I enjoyed and would make again. The yellow mustard I have is a little hotter than French's, but still mild enough that I would happily lick it off a spoon. I started with about as much mustard as the recipe calls for, and didn't find the need to add more as the mustard flavour was strong enough, though without heat.

Mustard freezes, by the way, divided into small portions.