Help me finish-I simmered chicken in chicken stock, Rosemary, I added Dijon mustard & cream..what else?

For next time: reduce the liquid by about half before you add the mustard and cream. And yes, I'd include some dry vermouth or white wine in the liquid for the simmer.

You probably served it hours ago, but if you feel it needs more than those ingredients:
salt and pepper

And you need to allow some time for them to sink in, VERY low heat, so as not to over-cook

I make something similar adding fresh herbs, I like basil and parsley and cherry or grape tomatoes. Eat it over egg noodles or even rice. I agree with Susan W an acid like white wine would do nicely.

And serve it over some pasta!

I think it needs some acid. Lemon juice, vinegar, dry white wine? Also, salt and pepper if you haven't added those.