Yeah, it's vinaigrette made with the bacon fat & some sort of vinegar.
That is its simplest form.
The more complex version, that is something really special, goes something like this:
Cider vinegar (1/2 cup or so?), mustard seeds (a couple tbs), maybe half again as much water and cook until the mustard seeds are rehydrated- this takes around 15-20 minutes. Then strain out the seeds- keeping both seeds & cooking liquid- but separated.
Bacon in a pan, add sliced shallots, cook until the bacon and shallots are brown and crispy... stirr occassionally. Break up the bacon... or you can chop it before adding it. You want to do this over medium- medium high heat- but you want it to render out nicely and not burn.
When bacon and shallots are nice and browned add the mustard seeds- they'll pop after about a minute. Add in the other liquid and a bit of brown sugar. Maybe a tbs. Taste... add salt/pepper to your liking.
You can also run anywhere on the spectrum between the two. If the whole mustard rehydration bit seems OTT to you, just get some stone ground mustard. You can make a perfectly respectable dressing by simply making bacon and using the bacon grease to make the vin, stir in a bit of mustard and brown sugar, and off you go. I like apple cider vin, and I think this is a great place to showcase really good brown sugar and bacon. But you can use whatever you have on hand. I think the shallots add a nice flavor you can't really get with other onions, but you can skip them or sub them out (you could also use sweet onions). You can mix things up with different herbs as well. Terragon mixed in at the end is nice, maybe rosemary... you can mix it up.
It does have to be 'warm' though because bacon grease solidifies at room temp.
