Reading the recipe, it sounds all too rich and odd. Marsala wine is
very intense and I only ever use in small quantities like a tablespoon
as one would with say port wine.
Improvising for this recipe I would have deglazed the pan with 2 tbsp
aged sherry vinegar (or a tablespoon of Marsala if you must) , add
peppercorns and more mustard (2 tsp?), scrape pan and add maybe at most
3/4 cup beef stock. Add any meat juices from the steaks keeping warm.
Then reduce down by 2/3rds and add a 1tbs cream to thicken, season and
serve.
Goodness, that sauce recipe above would make enough sauce for a
restaurant!!

Just my take on this.
Cheers
Bronwyn
Oz