Marinate the liver in a mixture of the vinegar, Worcestershire, dry mustard, garlic, salt, and pepper for 30 to 60 minutes.
Remove the liver from the marinade, reserving the marinade. Pat the liver dry and dust it with flour. In a heavy skillet, over a medium high heat, sear the liver in a little of the reserved bacon grease. Turn the liver only once to avoid overcooking. Remove the liver to a platter and cover with loosely tented foil to keep warm.
Drain the excess bacon grease from the skillet. Place the reserved marinade and chicken stock in the skillet, over a medium high heat, and reduce by half. Pour this sauce over the liver and serve with the crisp bacon slices and pickled serranos or jalapenos on the side.