In a saucepan over medium-high heat, bring the cider to a boil and let boil until it's reduced down to 1/2 cup.
Now whisk in the mustard and cream; season with cayenne, salt, and pepper.
Continue boiling until this mixture reduces down to 1/2 cup.
Remove from heat and keep warm until ready to serve.
You could prepare this in advance and reheat gently when it's needed.
I have made this sauce only once--I don't know why I haven't made it since--and I was impatient and did not let it reduce the second time all the way down to 1/2 cup; it was still rich, luscious and very, very yummy.