Can I fix butter (for a sauce) after putting too much mustard in?
Simply do not use the butter for this dish. Cordon bleu does not require butter, only cheese and ham.

EDIT:

@Joe thinks you mean a mustard sauce to go with the chicken cordon bleu. Is that correct?

OK so in the absence of your verification or any other feedback, the answer is: 
you need increase the other ingredients so the ratio butter : shallot : mustard is back in proportion. 

For example the recipe calls for

1 Tbsp butter
1 shallot
2 tsp mustard.

but you put in 4 tsp mustard by mistake?

You increase the butter and shallots so it is all double quantity:

2 Tbsp butter
2 shallots
4 tsp mustard.

1 : 1 : 2

2 : 2 : 4

3 : 3 : 6 etc.

On the other hand you can simply just ignore the extra mustard. It may need a little more liquid for the right consistency because there is more starch than usual, but basically the sauce will be just fine. Do not add extra salt before checking the seasoning. The Dijon mustard I use is very salty so I never add any anyhow, but YMMV. 

It would be nice if you could acknowledge our various replies, Linda.