chicken sauce out of morello cherries?

I like Antonia James's idea (and also your idea for adding mustard) very much; just wanted to add that I think a splash of sherry vinegar or even balsamic would go nicely with the cherries as well.

Sounds pretty good to me. I've never made that, nor do I have a recipe, but I would probably coarsely chop (maybe cut in half) the cherries, depending on how large they are. I'd first finely chop a shallot and soften it in the pan after removing the cooked chicken breasts. Then I'd drain and lightly rinse the cherries, as the syrup would make the sauce too sweet. Some fresh thyme would go well, as would a splash or two of wine and some butter. I like the idea of adding mustard!! A bit of stock would help, too, as you should simmer it all slowly for 5-10 minutes, to bring the flavors together. Add more water or stock as necessary to keep it from drying out, getting too thin, or reducing too much. I'd probably throw in some finely chopped parsley right at the end, as well as a splash of red wine vinegar or a good squeeze of lemon, to brighten it up. ;o)

I make a raspberry reduction to go with Pork, similar flavor profile here is the recipe http://food52.com/recipes/12997-pan-roasted-pork-tenderloin-with-raspberry-reduction
I just use shallot, white wine, butter it's delicious and I think would go nicely with the cherries instead of raspberries.