It's all gravy: Pork tenderloin with mustard cream sauce

I recently invented this fine sauce. I had a pork recipe that called for a sauce made from port and cherries. But that sounded a little overblown and SWEET, so I came up with a mustard cream sauce instead. Try it on seared pork tenderloin!

Ingredients
1 onion
3 T grainy mustard
1/2 cup heavy cream (or more)
1 T green peppercorns (the kind in a jar)
salt & pepper

Assembly
Ideally you will have a pan in which you have just browned your pork (and then possibly you put the pork in the oven to finish off). Put the pan back on the burner and saute the onions until they've started to soften and brown.

Turn the burner down a bit and add the mustard. Stir. Then pour in the cream. Stir everything around, making sure to scrape up any savory bits stuck to the pan. Add the peppercorns and salt and pepper. Taste. If it's not punchy enough, add more mustard. If there's not enough to go around, add more cream.

This sauce will hold for a good 15 minutes over low heat. The cream will evaporate more & more though, so if your sauce starts to disappear, add a touch more cream, or even some water, and reheat right before you spoon it onto plates.

I'm sure this would be divine on other meats too. Thank you dear pigs.