What can I add to half-fat creme fraiche to make a sauce

[Feistybird]

that will go with leeks, mushrooms and brocolli?

(to be eaten with lemon sole).

Lemon juice?
Whole-grain mustard?
Cheese?

herbs/black pepper

roule cheese

ooh thx for these.

(Looked about an hour after I posted and there were no replies - was just about to bump)