I served this to guests who flew in tonight; they were so hungry (airline peanuts just don't do the trick). The sauce was so delicious on the mashed potatoes I used as a base, and on the spinach as well. I think I might use add more beef base next time and season the steaks with salt, but our guests loved it; there were no leftovers. None of us had used a mustard-vinegar marinade on steak before, but it gave a great flavor overall. Also, the presentation was great -- quite impressive my guests said. Thanks for posting this!!