Incredibly Good, Buttery Mushroom Topping

Anyone that's ever been to an expensive steak place knows that one of the best things in the whole world to put on top of a lovely chunk of perfectly cooked meat is a beautiful compound butter. For the purposes of this, I will pretend "compound butter," which means something quite specific, means "flavored butter." Since "compound butter" melts directly on the hot, grilled meat, I will further take "flavored butter" here to mean "melted flavored butter," i.e. "butter-based sauce-like topping." If I had called this post "Mushroom Butter-Based Sauce-Like Topping," it would have lost 98.3% of its appeal (I'm a mathematician, so I made that number up just like any regular person could have done). This is, in fact, very similar to something I did recently, although I took it in a slightly different direction this time and made something lovely, proving that creativity often lives in variations on the uncreatively reused ideas from last week. Then I decided it needed its own spotlight. Too bad I'm not good enough at Photoshop (and don't own it) to put a spotlight shining on the picture I took of the sauce-like stuff cooking. Thus, imagine there's a spotlight, and let it be as glorious as you know this thing tasted!
There it is, bubbling away. There isn't too much in there:

• three white mushrooms, halved and sliced very thinly;
• a clove of garlic, smashed, chopped finely (first) and added last;
• an "edge" of a medium onion (1/4 c.-ish), sliced thinly and halved (about an inch long and paper thin);
• three or four tablespoons of unsalted butter;
• salt and black pepper to taste;
• about three tablespoons of nice red wine vinegar (substitute your favorite flavor of vinegar depending on the dish you're making).

I didn't put any fresh herbs in this because, honestly, I didn't feel like going out to where they're growing and picking them. Thyme would have been particularly nice, though, as would rosemary or whichever herbs happen to be the best complements to either what you're putting this on or what side dishes you're serving. I was just putting this on rather uninspired burgers (cumin and crushed black pepper crusted, actually, served on nice thick toast with mayo and hot sauce). This would have been spectacular on top of any of the following, though:

• steak (wow, yes);
• burgers (I know; I did it);
• meatloaf (hmm... that's a better idea than I thought when I decided to type it...);
• chicken (baked or grilled);
• pork chops or roast;
• sliced roast beef (also known as "Reason Number Three not to be vegetarian");
  
• pasta (if I doubled or maybe tripled the recipe, particularly if I did it with thinly sliced grilled chicken and fresh chopped tomato added at the last second so it barely warms up and doesn't cook);
• scrambled eggs or a scrambled egg burrito if you don't mind it dripping with buttery awesome.

The making of it was easy also: melt the butter in a small saucepan or skillet, add the not-garlic veggies and salt, cook until the onions go translucent, add the garlic, cook for 1-2 more minutes, add the vinegar and pepper, and let it reduce (i.e. concentrate and marry the flavors) for 5-10 minutes over medium heat or until whatever else you're cooking is done cooking using lower heat as needed to accomplish that feat. To serve it, proceed to put some including the sauce it's kind of swimming in over whichever of the above things you've made. If you're doing it with pasta, just put all of it in the pasta and toss it.